you own the filters you create. that's stated in the filter submission agreement. you hold the copyright. however, BY SUBMITTING TO THE LIBRARY you are licensing to FF the right to publish and re-license to the end users, the right to do almost anything to those filters, including the presets, and including making derivative, protected works, and to alter the filters and re-submit these as your own.
the ONLY restriction FF puts on this is the right to pull those filters it deems to be of a spurious or negligible change to the original. in other words, if you change something fairly simple and re-submit this minor change as if it were your original work, FF has the right to pull that filter from the library or disallow it ever being entered.
legally, you NEVER have to give credit to anyone for using a portion or all of their filter(s) in yours. legally. ethically and socially, it is the right thing to do and there's a note about this in the wiki help pages.
if you do NOT submit your filter to the library you are still the copyright holder of that filter, just as you are when you submit to the library, and as such, may make and use your filters in any manner you wish, including publishing derivative works or even in selling your filter (good luck on that one

).
you also, per the submission license, have the right to request of FF that your submitted filter(s) be removed from the library and they will do this. but they retain the right to maintain an archival copy once you've submitted a filter to the libary, even if you ask them to pull it later.
NONE of what i've just said is legal and binding. this is just what i remember of the various licenses, eula's, wiki helps, blurbs on the FF web site and discussions with vladimir on the forums about this subject. so, DO NOT TAKE MY WORD FOR ANY OF THIS. READ the licenses!! GO LOOK THEM UP!
and my personal opinion about someone being upset with someone else 'ripping off their filter' is, USE THE CONTACT FORM and WRITE FF! ranting and flaming and blaming and whining on a public forum just makes life for all of us a grievous mess and frankly, we dont want to hear your accusations, accurate or not. send it to the folks that can actually DO something about it and keep the boards clean!
and again, in my opinion, the licensing FF is very smart and very generous. they maintain the engine as a protected piece of property. but, the SUBMITTED filters ACT (not that they actually are), but ACT as if they were open source, to a large degree. they arent open source, but that's the feel of it to me. and i like this. this is freedom and free exhange of ideas and creative talent. it promotes an exhange of ideas and creativity. and it's been working pretty damn well as evidenced by over 3000 filters being submitted.
so, encrypting the filters, 'locking' things so others cant use them and all of the other ideas i've seen for curtailing use by filter makers is, to me, wrong. FF's strength is this open source feel, this free exhange. it works.
that there are those that might want to gain a freebie copy by doing a quickie job of alteration on an already successful filter, yeah, that's going to happen. so, notify FF when you see this or suspect it. the folks at FF are far from naive. they'll recognize one of these when they see it, just like you did.
that there are folks that are going to use parts of other's created filters to make their own and not give any mention of this, yep, that's going to happen too; sometimes knowingly and unknowingly.
but there's also a limited number of components in the editor. when do you claim that someone borrowed your idea? where do you draw the line? i plug a threshold into a set alpha, you plug a threshold into a set alpha, is that 'borrowing'? so, where do you draw the line? see, there is concurrent creation also. i've seen this more than one time. someone in china creates something almost exactly the same as someone in europe and at almost the same time. but neither knew or had any contact with the other. it happens. the same can happen with filters.
now, from all this you may suspect that i ignore when i really do borrow someone's filter or a portion of it. i dont, at least not when i submit a filter with a borrowed routine in it. it costs me NOTHING to say 'and thanks to steviej. i borrowed a part of his Windex filter here.'. steve gets some applause, i get some applause, and everyone gets a new filter. if FF thinks i've done only a minor change, they can pull it. when i think it's only a minor change, i usually just post it on the forum rather than the libary, with the intent that steve can use it to update his filter, if he wants. again, everyone wins.
so, yes, it's polite to give credit where credit is due. but no, let's not start closing up the architecture of the FF filters to 'protect us from evil'. that's the wrong way to go. this world is built on 'borrowing' ideas. we see something, get an idea from it and make something new. you wouldnt be on the internet here discussing all this if someone hadnt 'borrowed' someone else's idea to build something new. we'd still be back in the stone age. the statement of 'standing on the shoulders of giants' is very apropo to FF and filter making. going all insular and paranoid just isnt very workable. you might as well shut FF down right now. this works because we DO share ideas.
and frankly, though voldemort doesnt ask for any credit, i say give him credit when you borrow something of his. he's more than due!

If wishes were horses... there'd be a whole lot of horse crap to clean up!
Craig