It was offensive because it was aimed at CHILDREN. Kids are cruel enough without giving them more ammunition to use. I can just imagine a 10 year old redhead in school having one of these cards left on his or her desk. Blonde jokes are aimed at adult women, the card was aimed specifically at children. What if the card said "santa loves all children, even ones with afros". Would that be OK?
I'm not a redhead but my daughter is, thankfully I don't live in the UK anymore where it seems it's OK to take the piss out of children. People stop me in the street and tell me how beautiful her hair is. I just hope that she is always seen as unique and not as a target for bullying because she is different.
Had this card been directed at adults, I wouldn't have been so upset by it, but come on, kids have enough to deal with these days.