The first half of your sentence is right. But Rice will lose. Private organizations are under no form of compulsion against layered punishment. Lots of guys get punished by their team and the league, for big PR hits like this it’s basically the standard, teams often don’t want players that draw that kind of fire. Pacman Jones got cut by the Titans while suspended by the league, Rice has got nothing to stand on.
Rice's contention is that the league imposed a suspension of six games, then imposed a second punishment of an indefinite suspension.
And according to Rice's lawyers, the league is only allowed to impose one punishment per infraction.
It's not the punishment that is the issue so much than the second punishment that followed the first.