Left wingers. like all classes of people, think that their views are normative and applicable to everyone.
Never mind that in um, say, the last British general election, it was a SECRET ballot, and that in the privacy of the voting booth there is NO pressure or fear of reprisal and the ordinary voters can cast their ballot as they think.
"Some people have normative political principles and sometimes those principles aren't winning ones at the polls" is a normal thing to notice, sure, and annoying because of what a pointless statement of the obvious it is.