I see this viewpoint a lot. Nobody should feel obliged to gift at all, and teachers really don't expect anything. However, many see teaching as more than just a job and therefore wish to pass on a token of their appreciation. That person is caring for your child day in day out (likewise the TAs). They are helping to shape their future and become a massive part of their life over the year or so that they're in their class for. If you feel they are doing a great job and appreciate them for that, I don't think it's unusual to want to give that someone a gift at Christmas to say thank you, if you can afford to.
We use to gift the carers at my Nan's old care home at Christmas. Some people gift to midwives or nurses after being under their care for a long time. Nursery staff play a similarly important role in children's lives, and many chose to gift to them to thank them for that. These people work in jobs which don't come with Christmas bonuses from the big bosses. I know not all office jobs do, but many are rewarded in such a way. Or some office based workers will have clients who gift to them, as a token of their appreciation. For people in caring professions, thank you gifts at Christmas are their 'bonus' in a way.
Would you not tip a taxi driver/waitress/hairdresser etc because they're just doing their job? Or do you tip to show you appreciate a job well done? Many choose to gift to teachers and Teaching Assistants (who are paid an absolute pittance) at Christmas to say thank you if they feel they're doing a great job with their child.
Basically, nobody should feel obliged to gift to anyone at Christmas. No teachers, nursery nurses, carers etc expect gifts, but they're always most appreciated. Many people in such roles go above and beyond with very little thanks at times, so Christmas is a time for people who appreciate them to reach out and say thanks for everything they do for them or their family member.
Of course, there will be many teachers/carers/nurses etc out there who people don't have such great experiences with. I'm definitely not suggesting people should gift to everyone in such roles, just for the sake of it. If they feel their care warrants it though and they can afford to, why not? Many will go far above just 'doing their job', so it makes sense that some people wish to show their gratitude through a gift at this time of year.