We also use singular words when referring to the plural. Corn is a perfect example. Corn is the singular and the plural.
So using "them" when referring to corn (or in this case popcorn) makes sense. There are multiple kernels and with "them" being a plural pronoun it fits.
No? I don't buy handfuls of popcorn. I buy a bag of popcorn. It's a single transaction for a single line item. Nobody says "them". Why are you still trying to defend this? People do not talk like this.
You can't say "one popcorn" or "two popcorn" because mass nouns aren't countable. It's just "popcorn" for any amount of popcorn. Notice I said "amount" and not "number" because, again, popcorn is a mass noun and cannot be enumerated. If you want to enumerate kernals of popcorn, you have to say "kernels of popcorn".
A serving is often treated as singular a unit in English. Popcorn, rice, candy, etc. "I ate all of it," not "I ate all of them." Only when referring to pieces of popcorn does it become them.
Well, most couples would share one bag, and in this context specifically, it would also be awkward wording even if that's what they meant.
In the first panel he cashier is asking if they want 'them' salty or sweet. Indicating that contrary to what would be common this couple has, indeed, chosen to buy multiple bags. Perhaps there was a special offer making it make far more economic sense to have separate bags on the occasion.