Please decide if they are employees or contractors. Contractors can drive for somebody else.

At least half of the Uber drivers I meet are driving for someone else and that someone else is Lyft.

That's a whole other question.

The real answer is that they're a new category that doesn't fit into 20th century US employment law.

But until the law catches up to reality around 2048, I guess the category that's least wrong is "contractor".