You gotta go back and look at the numbers on the contract PIT was offering him. It was all about the guaranteed money. PIT never offered more than $20M in guaranteed money which was a low ball offer. Bell wanted more guaranteed money and PIT didn't want to give him that. The Jets did. Also PIT is notorious for being stingy negotiating contracts.
Here's an analysis of what happened financially:
Did sitting out 2018 pay off for Le'Veon Bell?
www.cbssports.com
The key point is that by choosing to sit out the entire 2018 season, Bell lost out on $14.5M in guaranteed money via the franchise tag, or $20M in guaranteed money if he'd have signed the Steelers' offer. And if the Steelers had chosen to cut him after just that one $20M year, he could then have signed that
exact same deal with the Jets.
Now maybe the argument is "what if he'd been injured during the 2018 season, and the Steelers had cut him?" And yes, if he'd been injured during the 2018 season, he'd have only earned $20.5M in guarantees for one season with Pittsburgh versus the $25M in two-year guarantees being offered by the Jets.
BUT, if he'd made it through that first season but been injured in his second, he'd have actually earned more in Pittsburgh than in New York, because the two year Pittsburgh total was greater than the 2 year NYJ total.
In other words, Bell's move only made sense if he was injured in 2018, in which case he would have earned an extra $4.5M. But if he wasn't injured at all, or was injured in 2019 rather than 2018, he cost himself at least $19M. And that's just a stupid basis on which to make a decision, because an injury in 2018 would have been no more likely than an injury in 2019.
A guy who makes stupid decisions that go directly to to his willingness to step on the field likely scared the shit out of potential trading partners for the Jets. That's probably why they had to release rather than trade him.