"They didn't like the founders/team, or thought the idea was really good, but missing a couple things, or they had a different direction they'd run it in. So idea theft does happen. "

My two cents:

You can't steal what isn't owned. If you think my idea is really good and then you act on it, you haven't stolen anything from me. I never owned that idea because ideas can't be owned. What if someone else three thousand miles away has the same idea? Do we both own it? Did you steal it from both of us?

On the other hand if, in your scenario, there was an established business plan, and other proprietary details that were taken, that's theft.

I guess it comes down to the details.