Ever tried telling your autistic son that you did your best to choose the right gifts and he should be grateful. Also ever tried telling him he can't have everything his own way? Surely autism can't mean that you just give up teaching good manners and all expectations of good behaviour? Any child could become unbearably demanding if boundaries are not set and upheld.
Autism is a disability. I know this is hard for some to understand, because in a way some NTs are far more rigid and inflexible than those who are autistic, so I’ll assume you’re not being deliberately inflammatory and that autism is simply outside your experience and understanding.
What you’re suggesting means that many autistic people would grow up traumatised and less likely to be able to adapt to situations.
These skills (including accepting a present you don’t want and didn’t ask for) are to be learnt. It’s a process, sometimes a very long one. If your child is autistic and you know they don’t like surprises, and that they may catastrophise for days before they can remember enjoying something, you put in place adaptations to ensure that they can enjoy their day as much as they can.
It’s not about being polite, and if teaching good manners is the goal then you do it in a way that the child responds to and that doesn’t destroy them in the process.
Suggesting that making allowances is letting them get their own way is ablist - just pointing this out so you can understand in future (apologies if this comes across as condescending, but I’m assuming from your post that you have a very low understanding of autism, so I’m trying to be very clear here).
You wouldn’t suggest that a wheelchair user should just get up and walk so they don’t take up so much room (I don’t know, maybe you would?), or that a child with a physical disability should not have adaptations to allow them to function or have a happy life?
Autistic people are the same. It’s a disability, not something we’ve acquired by being pandered to, or by having bad parents who didn’t smack the autism out of us. It’s a lifelong disability that we have to learn to live with. As parents we usually want our children to learn in a way that also allows them to thrive - I know, very modern ideas we have now! Making allowances throughout a day that help a child to accept the other things that Christmas brings is, believe it or not, a good thing.
Flinging an unexpected and rigid day with no consideration of an autistic child’s needs is a recipe for disaster. In these circumstances being “ungrateful” would be the least of your problems. It’s good parenting and allows a child to learn to accept other’s preferences and wishes to adapt Christmas so they can also cope with the day and enjoy it.
(As an aside, I’m often quite horrified that so many would prefer autistic children to suffer for other’s sakes. Almost like they’re not seen as human or deserving the adaptations they need to live their lives without trauma! It’s so upsetting!)