We should reject hard-to-drive speakers more often

If you buy stuff that doesn’t work well together, guess who’s fault it is ?

If a speaker is rated at 4 ohms but is actually a 2 Ohm speaker that is not the buyer's fault.

My point isn't about whose fault anything is. It's about what  we venerate.  "ooooooh, a half ohm speaker which needs a super amp... I must be mas macho if I get this working!"

...speakers need a warning label, like cigarettes and alcohol?

WARNING: THESE UNITS MAY BE HAZARDOUS TO YOUR POWER AMPLIFIER(S) AND YOUR BANK ACCOUNT!

Would this stop anyone?

Not likely....

yes Erik, the manufacturer providing false information is a very different issue than knowingly buying difficult speakers and not being prepared...

I personally have. Dynaudios are hard to drive, I don't buy them anymore, even though they are very high quality. Hard to drive speakers aren't fun at low volumes.

I also see this problem kind of hidden in reviews.  Something like this:

Speaker xxx was clearly able to discern the difference between amp A and amp b so it must be very resolving.

It wasn't at all very resolving, or discerning.  It was demanding.