Both parties have yet to deny that a guaranteed subscriber limit wasn't part of the negotiations. Scott pointed it out as well. That's from where my deductions come.
We don't know if that's part of the current contract, if that's an agreement HBO has with other providers, or if that is even standard in the industry. For all we know Showtime has a similar contractual clause.
HBO claims they offered to extend the current contract while negotiations continued and Dish refused.