It is more of a gentleman's agreement. We sign up for them, they in turn sign up for us.
Now if we have people signing up and the roster is full then they are free to do whatever they want (check the VE/LCO fight from last night Fcc had 5 guys on LCO who did rather well) , and same goes for VE when we are full up. It is all good. No harm no foul.
however in the case you are talking about. The fight popped up. It was at a bad time where NA mercs were going to be difficult to get. I look on the enemy roster and it has 100% sign up from the MB guys, that is def something we will try and change if we can. It is the politics of the game. No hard feelings but if we see people on your roster we would rather see on ours, we will happily try and get them to shift sides. Rosters are not set in stone till the fight begins.