In what might be an early attack in the owners trying to break the weakest union in sports, only one NFL first rounder has been signed; Dez Bryant. The NFLPA has accused the owners of collusion the last summer before the collective bargaining agreement is renewed. Caught in the crossfire are this year's first rounders, who want to use last year's draft pick salaries, while the owners force a slotting system that would cut rookie salaries.
In the case of Tebow, he cannot go to camp tomorrow if he is unsigned. Not only does he become a scab, he could become the invisible man in terms of NFL apparel and merchandise. By practicing for free, the NFLPA could remove Tebow's jerseys and likeness from video and fantasy games. The MLBPA did this to baseball players who crossed the picket lines in 1995 and trained with replacement players. (Imagine playing Madden with Broncos' QB Jim Jebow.)
I would think with Tebow's politically conservative leanings that he would not sympathize with labor over management. But this isn't UAW and General Motors. Tebow practicing without a contract may win him points with the Broncos management, but it screws over the other first round picks. ("If Tebow can come to camp, you can too.") He is also risking losing a source of income from royalties.
Tebow has to think about this like a businessman. The NFL is the only league without guaranteed contracts. NFL draft picks need to make as much money as they can today because it can all end tomorrow with a release. Tebow does not need to worry about that, but he needs to worry about himself and getting paid. Training camp will be waiting for him after he signs.