The issue isn't that someone can force you to vote a certain way or check that you voted in some way. The issue is that someone can make it outrageously convenient for you to make a whole set of uninformed votes.
Imagine you're very busy and haven't spent the time yet to figure out what to vote for. Someone tells you that if you vote a certain way, it will be great for a certain pet issue that you care about, and they give you (and many others) a QR code. The QR code contains a vote for one candidate who cares about the pet issue, but the rest of the votes in the QR code are all oriented around a different issue that you don't know about, don't care about, or actively care the other way about, but you don't notice and scan it and vote it as-is.
Imagine you're very busy and haven't spent the time yet to figure out what to vote for. Someone tells you that if you vote a certain way, it will be great for a certain pet issue that you care about, and they give you (and many others) a QR code. The QR code contains a vote for one candidate who cares about the pet issue, but the rest of the votes in the QR code are all oriented around a different issue that you don't know about, don't care about, or actively care the other way about, but you don't notice and scan it and vote it as-is.
Mail-in ballots don't have that issue.