I'd be surprised if calling something a "tip" makes it legally obligated to go to a contractor. I'm sure their lawyers are very aware of the law on this. As someone else said, there is almost certainly a binding arbitration clause. This removes the possibility of individual or class-action lawsuits.
edit: The abuse of the term "independent contractor" is just one of many examples of how labor law enforcement has become lax in the last several decades. How many people on this site aren't in management and work unpaid overtime?