Because sometimes "creatives" end up making something that's already been done, and legally protected. Just because someone hasn't "heard" of that previous work, doesn't make it any more legal.
Also because we're tired of seeing people who truly are creative and come up with something original(i.e. designed by themselves, not copying/duplicating someone else's work, ahem) get stepped on by the larger guys who just happen to be secretly(or not so secretly) holding the copyrights or patents. That, and there are more than a few lawyers that are members here and when something smells eerily similar to someone else's stuff, they know someone's gonna come knocking sooner or later demanding restitution.
How many people here know that not all the B/W '90's era games licenses are held by the same company? I figured they all would've been sold together, but that's not the case. From what I've learned here, I think Planetary owns a couple, BAA has a couple, IPB has a few, and the list goes on. I'm sure if someone wanted to make a new/replacement TAF part, however noble their intent, they'd get a call. Same goes for MM. Albeit from different companies.
Do you realize the number of patents and copyrights Apple applies for and gets every year for products they have NO intention of ever making? It's impressive. They do this to cover their asses on the technology they make that is largely similar and therefore can be extrapolated into other products that are very close in nature to the ones they already make. Basically, Apple doesn't want to be anyone else's R&D department. They put millions of dollars into their products, they're going to protect what they've got.
Back to pinball, how would you feel if you spent hours and hours designing something in SolidWorks, paid to have it 3D printed(not cheap), made a mold from it, reworked and refined the drawing and mold to make it just right, then started making small batches of 5-10 so you can sell them to friends in the hobby....only to have one of those people buy it, make a mold of your finished piece(that you spent probably a few hundred dollars on and about 40 hours of time/labor), and start cranking out what is essentially YOUR creative handiwork, just to sell it on eBay?
That's why we "bust out with all the lawyer crap"!