I had a Buffalo router that did that; IIRC it came with their proprietary firmware and a copy of DD-WRT on a CD. (Might have been the other way around; this was about a decade ago.)

I don't believe they would have been in much legal issues: they'd have to make sure the copy of DD-WRT they shipped was fine, but if you get updates / flash your own, there's no reason they'd be on the hook.