Hussar
Legend
Cor Azer said:1. That's copyright infringement, in all likelihood, and hence illegal.
2. Very often (not always, but often enough) prestige classes in supplements deal with other rules also in those supplements (Imagine trying to pull in a PrC related to a new magic system without the rest of those rules). That requires a lot more rules to be copied (see 1).
That notwithstanding, very often the rest of the supplement will have other tidbits that balance that PrC that may not be directly mentioned in the PrC (ie, opposing classes), or flavour details that speak to how the class relates to the campaign world at large. Sure, you could copy these as well (See 1), but as a DM who doesn't own the book, how do I know you got all of the relevant pieces? This isn't just a "I don't trust my players to not cheat" - I've played a plot of games with my friends, and sometimes we just accidentally overlook things - having the actual book in front of me (not just borrowing it) gives me time to read it thoroughly so I understand all of the implications.
While it very well might be copyright infringement, I highly suspect that photocopies for personal use in game aren't going to raise too many hackles. The vast majority of PrC's out there can be copied onto two or three pages and you're done. At the absolute outside, ten pages.
PrC's that have rules spread throughout a book are pretty rare beasts, unless you are using PrC's from something like Tome of Magic where the PrC is based on a non-core class to start.
My rule for my World's Largest Dungeon campaign is that any player who wants to use a non-core PrC or class or whatever in the game has to type it up and post it to the hidden password protected site we use for just such a reason. If anyone wants to get uppity about that they're welcome to, but, so far, no one has said too much. Despite the fact that I own none of the books that these are from, we have a pious templar, a Dungeon Delver, a Knight and a Favoured Soul. In the past we have also had ninjas and war wizards as well.
So far it has caused exactly zero problems.