Ok, as a mom that's had a bad day shopping before, I gotta defend this mom a little bit, only because she is not here to defend herself

First off, "piss" (classy choice of words btw

) is a relatively sterile liquid, and unless this child had a urinary tract infection, it was probably safer to come into contact with her own urine than it was to drink from the drinking fountain there. You could actually survive drinking your own urine if you were stranded somewhere.
Second, having a baby on your hip can sort of dislocate a diaper from being properly positioned. I have gotten peed on many times this way, only to realize that the diaper was still dry.
Third, I imagine this mom was rather mortified that she just got a golden shower (since we are throwing around tasteless terms and all) in public, so I imagine she was frantically trying to clean up the mess and did not put a lot of thought into what order she was cleaning it up. I imagine she wanted to make sure it wasn't dripping onto the floor.
Forth, well unless the child peed INTO her purse, odds are the gum was A-OK. Even so, many gum wrappers are rather water (and urine) resistant. She probably just wanted to shut the other little one up before drawing more attention to them.
Fifth, well as someone who cleans her purse regularly, not because it was peed on, but because I have children, and children + white striped Coach bag = MANY frequent cleanings, I would say that it's a safe bet that she just cleaned her purse because it was simply...dirty.
Ok, I'm done. I just had to defend this mom because I am sure most of us mothers of young ones have had many disasterous trips to the store where you just want to go home and bury your head for awhile. I know I have been out in public, carrying a newborn, and anyone who breastfed their child knows how the explosive breastfed baby poop is, and I have worn it in public. Not because I didn't change my child, but because that's just what those little Houdini's do.
Sure, its gross, icky, whatever, but does not qualify her as a bad mom