Compared to the other major sports, NFL referees only call one game a week and are paid less on average. Many of them have second jobs because of this. Shit, Ed Hochuli is a lawyer during the week and probably a professional body builder as well. Many refs are teachers, doctors, farmers, inventors, etc. So, the shield only offers part-time employment on a contract basis. Basically that means, the league can void a ref's contract when the season is over—the same way they do their players. Officials in the NBA, MLB and NHL work more games and therefore, are employed on a full-time basis. This is another major reason why the NFL doesn't want to pay the refs as much as the other leagues, which is understandable.
The NBA pays their officials anywhere between $100K to $300K, NHL officials bank between $72K to $255K (depending on if your a linesman or ref) and MLB umps make $120K to $300K. Not to mention, every official in every league usually flies first-class with all expenses paid. Other than being ridiculed for every call they make, NFL refs have it pretty easy. But, you can't fault them for trying to get that money, the NFL has enough to go around.