Running backs are different. They can be elite difference-makers, but only for a limited time. No one in the game takes the punishment elite, bell-cow backs do.
So the standard "I've got you for five years" crap doesn't and shouldn't apply for top drafted RBs who did their jobs.
The CBA is too broad a document to cover this niche concern, so these few elite RBs have to take it into their own hands.
Bell, Gurley, Zeke, Gordon... they deserve better than the CBA forced on them and so have played the only hand they can.
Otherwise it's too easy for a shrewd team to draft them high, burn out their elite years, let them leave as FA's and then take the compensatory pick thank you very much. The next team signs them for less and is sure to cut them, leaving them with whatever guaranteed they could bargain for.
Bell gave Pittsburgh incredible, world-class production and literally carried the Steelers to the playoffs. They went cheap, paid for it in lack of production and failing to make the playoffs. Fortunately Bell didn't let them pay him nothing for another precious year of his career. He also got to heal a full year with a young man's constitution. He made about 14.5 million in 5 seasons with the Steelers.
Bell's deal with the Jets was decidedly less than Gurley's, and he will be cut after 2020, leaving him with about 28 million and change. That's a lot on the surface, but for 7 years the guy comes away with 42.5 million and impacted his team more than almost any other player but Ben and AB (and probably more than the Jets' QB will).
DLaw's likely cut point in three seasons will still leave him with over 65 million, and add that to the 20 million he made his first 5 seasons in Dallas. Lawrence at over 85 million in 8 years clears double Bell's entire earnings playing just one more season (about the same if you count injury-missed games).
I like me some DLaw, but no way in Hell did Lawrence impact the game like Bell did.