The point about a full can as a weapon has nothing to do with the ring pull, you simply slip it into a stocking or sock which you have carried on board, and hey presto - and instant cosh with which you can do some damage.
If the can is open, that's not possible, because the drink will simply spill when the can is moved, so its potential as a weapon is neutralised.
But the lady's argument is valid - you cannot deny one customer an unopened can and then offer one to the passenger next to her - that is discrimination.