Shouldn't attending training camp be required (re: Brett Favre) to play?

It seems like the Brett Favre hold out was to avoid Vikings training camp a 2nd year in a row. Shouldn't it be required that you attend training camp in order to be eligible to play in the first few games of the season? It seems like if the guy had any leadership skills (which the QB is supposed to be a team leader) that he would go through the same requirements that the rest of the team has to go through.