[citation][nom]Morgan3rd[/nom]I'm unsure but maybe a poorly written bios could cause overheating or something like that.[/citation]
I guess it's mostly because of tech support. If you keep the original OS, then they can (theoretically) service your device, whereas if the OS is modified, it's out of their hands.
good thing they do not prohibit it, though, like other manufacturers. Nexus one has the same advantage, it does not prevent one from unlocking it, but by doing so, voids the warranty. basic legal precautions, after all.