I'm guessing he was tapeing a box and left with the tape by accident. That sucks, I leave with black crayons by accident probably every other day.
I have a hundred pounds of black crayon in my cave. Going to sculpt them into a statue. But they do say "here's a crayon"/"want a crayon?"; they're giving you the crayon. And who can tell the difference between a crayon I found and the crayon I was given?
Whether or not he deserves to be fired depends on whether or not he was actually stealing the tape. Of course, it's easy to fight the charge, who says it's official UPS tape? Preloaders are always searching for tape, who says no one doesn't carry some.
I'm guessing the center manager either saw him take the tape or is setting him up [over misloads].