Well that's similar to how he can't simply just choose United as his employer now, we need to find an agreement with Real.
The difference is that he was effectively a Real player last summer, as opposed to a Juve one. If he wanted to choose to be employed by Juve, they would have had to agree a deal with Real, which given the prices quoted, they were unwilling to do so.
Perhaps the fault on Morata's side was agreeing to that deal in 2014 or whatever it was. However, last summer Real were just exercising the agreement they previously agreed.