Because they are based on (sorta) imperial and nobody really knows the difference all that well. So people say imperial when referring to the non metric countries that happen to speak English.

At least insofar as being defined in terms of metric units, the statement you replied to was accurate for both imperial and customary.

Aside from being overly pedantic, which is the reason I used the term, the doubly overly pedantic reply is that we do have the Imperial system, just redefined to be different than the itself-later-redefined imperial system.

Furthermore, the names of the units are from the imperial system, even if the definitions themselves are not.

Try pouring an imperial gallon into a us gallon container and then tell me I'm being overly pedantic.

The names of the units are from the winchester system which is what predated both US customary units and Imperial units.

I didn't say "imperial gallon" I said "imperial system" which is both historically correct and what people colloquially call it. Yes, since we adopted those units the measures have changed repeatedly on both sides of the Atlantic. I get it.

The US customary units were 1st established prior to the imperial system existing so it is not historically correct to say they are based upon the imperial system.