In theory, this is correct. In the real world though, the story is quite different.

Nearly all high quality A/C recovery-recharging machines include an oil injection option. Usually, it's simply an externally mounted reservoir, and oil is added as needed. The reservoir doesn't require any special protection, and it's not a sealed unit. Any moisture that contaminates the oil in the reservoir is removed when the system is evacuated (as long as it's done correctly). Obviously, if the machine doesn't get used for an extended period, the oil in the reservoir should be replaced, but there's no reason to open a new, sealed can every time oil needs to be added to an A/C system.