Theft is the unauthorized taking of another's property; eminent domain is the power of a state to take private property for public use which means it's expressly authorized. Eminent domain is included as state power in the US Constitution, so considering eminent domain as theft means partially rejecting the authority of the Constitution.

I think that's well within the scope of this conversation, but if we're going to be worried about the conversion of a private golf course into housing then we should really talk about what happened with Native American land treaties.

tl;dr: if we're redefining theft then that opens a huge can of worms.