Why are people so mean?

There is a pharmacy in the town where my GP is.  When my daughter was little and just starting to go without nappies, she suddenly needed to go while we were in the pharmacy waiting for a script to be filled.  The (staff) toilet is off the main part of the shop but the woman refused to let her use it even though I said that I would clean up any mess if needed.  So I had to grab her, put her in her car seat and drive up to the local park, then run with her to the public toilet block.  We made it - just!  I won't say that I've never been back to that pharmacy (as sometimes I have no choice unless I want to drive down to the suburbs) but let's just say it's been about 6 times in the last 11 years and I can tell you, that's a lot of money they've missed out on.