The Logic is simple.
The Broncos trade up to have a CHOICE between Bell and Jones.
They took Bell.
I think we are all entitled to form an opinion about the Broncos abilities to evaluate RB talent. I believe HUGE injected Portis into this discussion.
When the Cowboys used their second pick, they had NO CHOICE. Bell was gone.
The Broncos should have contacted Huge before the draft and he could have explained to them that Bell fumbles.
I only suggest that the Cowboys MAY have wanted the option to draft Bell. The option the Broncos had when they traded in front of them. We will never know for sure.