NFL = No Fun League
I think the NFL is way too heavy handed in many of their dealings. However, I would guess that there is some type of agreement between the NFLPA and Reebok. I believe the MLBPA has some agreements with various companies regarding their products (video games for sure, but I would guess memorabilia as well). This is why Barry Bonds wasn't in any of the video games- he wasn't a member of the player's association, so the deals they made on behalf of the players didn't include him. I would guess there is some similar agreement in the NFL, and this may be the reason Johnson is bound by the rules here.