I think one major difference is that a reasonable default expectation is that when I have an ink cartridge in my possession, I am able to use it until it's empty. I wouldn't have the same expectation of the ink cartridge at the local copy center. I'm sure that HP states somewhere that the ink is their property even if you physically have it, as the OP said. However, I think whenever you violate reasonable expectations, you should also make an extra effort to make sure the consumer is aware.