Oh, agreed. Should have a way to punish someone for trying (e.g. attempted murder charges because the police stopped the murder and saved the intended victim). But even then, one still has to be able to prove it, and the level of proof - beyond a reasonable doubt - is as high as it is for good reason.

Now, if the accused had encountered police detectives at that polling station instead of real election workers, I imagine it would have gone like this:

(Police detective posing as a poll worker prepares an otherwise blank but non-obviously spoiled ballot.)

“Ok sir, here’s your ballot.”

Choice A: “Thanks, here’s the ballot, yay I just voted.” “Sir, you’re under arrest.”

Choice B: “Um… actually I already voted.” “Yes we know sir, I see it right here, but we were just testing you.” “No, hey, wait, I was trying to test you.” “…”

It’s not a reasonable expectation to ask actual election workers - poll working volunteers - to do anything like the above, though.