You are correct in the case of countries where legend drugs are available without a prescription, or where prescription laws are not enforced. However, in the US, Canada, and EU, additional labeling requirements are common, and the labeling requirements include the need for prescribing physician name, pharmacy name, address and phone number, and unique prescription number (in addition to the name of the patient). It is these elements that give a prescription medication package legitimacy. Carrying the prescription with you, as I advised above, provides an additional layer of assurance to the security agents wherever one happens to be, but to be honest I think it's only the US where one can successfully pass through an airport security checkpoint with an 8, 12 or 16 ounce bottle of lens solution dispensed pursuant to a prescription without too much hassle.

To the subject of medical necessity, there are many individuals who are unable to use commercially available contact lens solution, and either require additional buffering agents to be added to a commercially available solution, which would be done by a compounding pharmacy, or they require a solution compounded from scratch to be made specifically for them by a pharmacist. However, it would of course be possible for the pharmacist to prepare a solution in a bottle that is compliant with TSA and international liquid size requirements.