There's a lot a wiggle room in saying that your ink is almost out. The simple defense is that they don't want people to have wasted prints, which might happen if the ink for one color actually runs out, so they notify a little earlier. That "little earlier" is vague and relative, giving a lot of defensible ground for a lawyer to work with if it comes to that. Stating you are selling a certain amount of something and not actually delivering that amount is illegal, and fairly cut and dry at that. It's not hard to see why one is more likely to be attempted than the other for a company.