Make your snippets free as in beer, add a line that says "If this is helpful, let us know. If you do a modification that might be useful to someone else, you will improve your karma by making that mod freely available, just like these are."
The people who would observe the license clause about open-sourcing their mods will release the mods back anyway, if you just ask. Honest, ethical people are honest, ethical people, who do the right thing even when nobody's looking. The people who'd keep their mods private will do it anyway, whether the license allows it or not. Scumbags are scumbags and will always be scumbags, regardless of who's looking.
You can claim the copyright (if it is in fact yours to claim). That and a dollar gets you a cup of coffee. That and a whole lot of dollars lets you sue one (1) scumbag, and you'll probably never recover anything like the cost of the suit. Meanwhile, ten or twenty of his brother scumbags have crawled all over your site and are doing exactly what he did, and you then get to spend ten or twenty times as much money suing them as well.
Sometimes, you're better off taking the High Road and building good karma.