to kako
This is true of everyone not ashkenazim.
One (anyone) may only know what the common practice
is and not if it is
a chumra
a local minhag
a minhag of most or all
a din d’oriaisa
a din d’rabbanan
Current accepted practice that may have
been matter of machlokes in the past
And so on.
Why do you attribute such lack of
knowledge particularly to ashkenazim?
in your example, the person who thinks that something actually practiced by many outside his community is assur, probably doesnt know that that is the case, or that the shulchan aruch permits it.
And don’t be too sure that you may not have some similar lack of knowledge about something.