The NFL doesn't have guaranteed contracts like other sports. The NFL is arguably the most physically dangerous of the major sports. Bell played on the tag in 2017 and didn't sign the 2018 tag, which I have zero issue with because one knee injury would jeopardize his ability to earn a deal like he signed with the Jets and could have ended his career like Shazier's career.
If Harrison was told he would be played, but wasn't played as he was told when he was signed....I again have no issue if he snored in meetings or threw tomatoes at the screen in film session to force a way out. Let the man go if you weren't honest with him in the recruiting process.
The circumstances of players leaving is really not a big deal to me. I just find it hilarious how some Steeler fans totally discredit a players ability and performance because "he left us, or he quit on us". Especially when they are just doing what is best for their personal career.