Oh, and I take it that when our DC are adults, we are no longer allowed to research and choose a gift for them?
Clearly when they are 16 they should be out earning their own money and buying their own gifts otherwise how will they ever be able to cross the road without holding mummy's hand... 
This place does my head in. I bet most of these posters have primary school DC and have no concept of having teenagers or adult DC.
Now that I've done the hard work of raising DC to know right from wrong, taught them how to swim, cycle, drive a car, cook, take care of themselves, know basic first aid etc. I quite enjoy treating them as I would a friend, i.e. doing them favours from time to time, being kind and considerate (and expecting the same in return), and buying them gifts from time to time. I like them, I enjoy spending time with them and they seem to be okay with popping to the pub or away for a weekend with us so I'm guessing they don't mind us either.
I'm sure if a uni bound DC doesn't like the bag purchased, they'll either be polite and say thank you and use something else, or say that it's not their cup of tea and ask if they can exchange it. It's not like OP or anyone else is insisting they go with a Thomas the tank engine back pack is it? And at the end of the day, I hope I've raised my DC to not really care what everyone else is using and to be happy with something that suits them and they like.