They do it to try to "incent" you to buy one of their Superdrive equipped Macs. They actually charge very little for iDVD, the balance of the software development cost is made up in superdrive sales.
In fact, I read that one manufacturer wrote their own patch and started distributing it with their DVD-R drive, and Apple's lawyers made them stop.
I would assume there are probably hacks and patches out there somewhere. No idea where though.