That's really strange. I wonder if she saw Sprout? Did she mistake the situation, and think that Sox just was some random adult wanting a balloon for herself, like the elderly lady before her? Maybe she thought, "Geez...why are all these adults taking up my balloons for the kids?! I need to end this here. I'll give one more adult a balloon, and that's it."

And no...it's not petty. It does sound weird. I would have been very put off as well.