Printers definitely are super annoying, and I don't want to discount the catharsis as a factor in why this would be satisfying, but one point I don't see made yet is that independent of their ability to induce frustration, they're just full of "crunchy" bits that would give really nice feedback to being smashed. I think smashing things that are too soft would be unsatisfying, but obviously if something is too hard it would be potentially too difficult to smash effectively; printers seem like they'd be right in the sweet spot of just-hard-enough-to-smash, and the presence of all of the small moving parts provides some nice low-hanging fruit in the early stages before you fully break through the outer shell. Some comments here also talk about smashing cars, and I think they're similarly "crunchy" in a way that would be super satisfying.

tl;dr it's not just that printers are awful, but that they're crunchy