Well, I went to my local shop Maximum Scuba on Richmond in Houston. They told me that they need to let all the nitrox out. When this will happen it will be empty. All empty tanks need a visual. Thus, they will fill with air only if I will shell out for a visual.
BTW: The tank is new, visual was made 2 weeks ago.
Sadly, everyone telling you to get a new shop is right. Not only do they not need to get all the nitrox out, they don't need to do a visual when they do. The reason you were told above that you are being "hosed" is that it is extremely hard to believe that people who make their living in part from filling tanks could be so terribly ignorant, so the assumption is that they are lying to you. You don't want to patronize a business that is lying to you. On the other hand, if they aren't lying to you, then they are terribly ignorant. You don't want to patronize a shop where they are terribly ignorant, either.