Besides, aside from it being more beneficial to others, they may only have a limited amount of the good candy; once they run out (or at some point in the evening) the mark becomes useless.
A good indicator is often how elaborate the display is, although it's not guaranteed. But it at least shows they're in the spirit of the occasion and have probably bought the good stuff so they can enjoy it themselves as well. Stores were also good places; at least one store would give out cans of pop. (You need something to wash down all that candy and chips.) Another would give out full sized candy bars.
