Most loaders that use the lock out bar, have a bar of a different color, I asked a dealer one time, he stated it was a safey feature to be of a different color, so you can observe that is is either engaged or not at a glance. I do not know if that is true, but it would make sense if it is.
Yes, I'm going to say it's a requirement with a non-removable loader, if you can take the loader off and service that would be the best, but since the M59's loader can't come off, along with full sized TLB's and such, that can't have the loader come off are required to have one of those for safety. They have them on all of them without removable loaders.