Here's the contrast: The Steelers don't have to trade him if they are not happy with the compensation. This is why Colbert has the leverage here.
The announcement of the trade wasn't for the league's benefit. GMs call all the time to inquire about players. This announcement was for Bell's agent. Hey, you want to get paid this year? Find a partner and play ball. Hell the agent can even save some face by finding a trade partner willing to pay Bell his entire full season tag value, and that is a ******* win at this point. Steelers aren't dropping him for nothing, Bell isn't getting a NEW contract this year, so the agent's only recourse is to get more money at this point in the season, than would currently be guaranteed on his tag.