One set of words that may be well in this discussion is:

Ass-burgers people.
When you are a normal person, you understand this give and take of gift-giving.
You understand the social construct that is the exchange.
And since you are a normal person, and they are a normal person, you just naturally do the whole gift thing.

However, people on the spectrum do not understand this social construct.
In fact, their lives are often in direct opposition to it.
As in, so often it is painful to receive a gift.