It is also enirely and completely useless. You would get desensitized to the warning the second or third time you buy grapefruit, and have 0% chance to remember the ubiquitous warning when you actually get some medication that does interact.

The important thing is for your doctor to be aware, and notify you.

Also, grapefruit is nowhere near the only food that can have terrible interactions with certain drugs.

Well, ours is the profession from which "RTFM" arose as jargon (admittedly not without justification) so it doesn't seem that strange.

This was literally the argument for Prop 65 warnings