My son is 6, and we do not supervise him in the bathtub. We have not for at least a year. He likes to 'swim' in the tub, put his head under water, and who knows what else. He will sometimes be in the tub for 30-40 mins until I holler at him to get out. He has recently become an excellent swimmer though through regular swim lessons. That means he's around a pool a LOT!
Roughly 500 children under that age of 5 die each year from drowning in a bath tub. There are 300 MILLION people in the US! If people are really that concerned, then they should not even step foot near a car (800 THOUSAND die each year under age 18) or go anywhere near a swimming pool (700-800 per yr) - oh, and if you're potty training, be sure to sit next to them, so they don't drown in the toilet (15-20 per year). Then there was the toddler who drown in the wash machine recently. That might even be a first!
I'm not saying be stupid and let your 2 year old bath while you go out for a jog, but I am saying many people are being overly paranoid. Our children need to live a little or they will grow up not knowing how to make decisions for themselves (like many teenagers right now). No, I don't think there is anything those parents could/should have done.
Just my $.02.