The fact that Catalans are in France, Catalonian national aspirations notwithstanding, means that in spite of SL membership they are still subject to the French RL and should be releasing their players for France and, indeed , it should be compulsory for them to do so just as English clubs must release their players for England.
And Catalans Dragons do release players for international duty. But if those players decline to make themselves available for selection due to injury, surgery or simply because they feel they need a long off season, there's precious little the French Federation can do about it. In just the same way the Welsh team are without several SL players who for whatever reason are not available. You could then shift the blame onto the players themselves, but perhaps they might ask more of the games authorities who seem happy to have players playing an ever more demanding game for 10 months of the year.