Though you're legally allowed to, and, we like the idea of any one running a server using our product of course, there are a few out there who might not like being 'forced' to use a certain product (even if said is 100 times better than any other

).
So, yes, your more than welcome to, but, you might have a few customers who dislike that idea, or a few potential customers turn away.
Make sure to give people choice, even if it's an 'opt-out' checkbox during purchase of your hosting.