Many seemingly inviolate rules of English are giving way to expediency. If you ask someone if you can get her something from the coffee shop, everyone understands what you mean. Of course you are able to get her the coffee. May you? Are you allowed to? Is it permitted?
These are silly distinctions, in my opinion. If your meaning is not open to interpretation—whether you use can or may—be my guest and use whichever word is natural for you. No one will deride you for continuing to make the distinction, but neither should they tsk tsk at you for ignoring the old rule.
We have to recognize that the English language changes. All languages change over time. If you are not comfortable with prevailing usage, stick with what you are comfortable using. Sometimes there is no clear “right” or “wrong.”
Feel free to disagree.