In fairness to the Yankees (WTF am I saying?!), that's a much more cut-and-dried situation. You can't play dangerous sports or hobbies while under contract, because you can get hurt. Everyone understands that, and like everyone says, it's black-and-white and inarguable. A few of them go skiing in the winter anyways. No one really cares unless you get hurt, but if you do get hurt, you should expect them to throw the book at you, because you selfishly let the team down and, from a business perspective, deprived them of the value of their bargain. You have some sympathy for people just playing rec basketball in a gym, trying to stay active in the winter, but he's also being paid millions of dollars not to do so.
So yeah, that's another example of the Yankees voiding a contract, but I don't think it falls into the category of "sin of not being as good as they thought he was when they signed him".
As for Ellsbury, based solely on the reported facts of "sought treatment from a licensed doctor who may or may not have had a professional error 8 years ago and is clearly being smeared by the NYDN", I have trouble feeling like this is anything except sour grapes by NYY management. Was Ellsbury taking PEDs? Is there evidence? A failed test? No? Then the complaint is that there are "suspicions" about this doctor's medical practice. Come the hell on. Absent any shenanigans with prohibited substances, all this proves is that Ellsbury was sincere about trying to get back on the field.