I've always known a "gallon" paint can actually contains a little less so they can add tints, and I'm surprised this seens to be a news flash to so many people. Even if they charge the same for a full gallon and the "shorted" gallon, that makes sense too. If you don't need the paint tinted, you buy the full gallon. Normally you don't get charged for the tinting, so even though you buy less paint initially, the fact that you end up with nearly a gallon, and the work involved in tinting it, to me makes it worth it.

Im sure if they were given a choice, the paint store would rather sell full gallons pre-tinted instead of having to tint it themselves. Not only is there labor involved, but the tinting process is not fool-proof. They have the hassle of dealing with customers who keep bringing the paint back for "just a smidgen lighter color", or trying to match another batch of paint because the customer didn't buy enough originally (almost impossible to do precisely). Not to mention the store has to absorb the mixer's mistakes (oh you wanted color #2718? I thought you said #2719...)

Bottom line... I think you're way off base here.