When people claim they're legally "obliged" to enforce their IP, they're saying that if they don't enforce it, it could possibly be argued that their ownership

When people claim they're legally "obliged" to enforce their IP, they're saying that if they don't enforce it, it could possibly be argued that their ownership rights are diminished because the work was orphaned, just as abandoned physical property can lapse to the city if the owner is absent and the property unmaintained. They're not actually "obliged" to do anything with their property. Most of the time that argument is a convenient cop-out. You'd have to abandon your trademark pretty hard to lose the rights to it; leaving a few obscure online projects unmolested won't matter. The real deal is that companies are very protective of their IP because they don't want anything to potentially impact their brand(s), and lawyers are eager to bill for as many hours as they can. It's possible that IP owners could commit a small team of goodwill ambassadors to filter alleged infringements and send licenses instead of (or quick on the heels of) C&Ds to good projects, but who's going to do that?