I agree with Hecate and Edam on this one.
Yes, in an ideal world everyone would accept all differences, there would be no racist/disablist/ageist/sexist attitudes at all.
and our children would be safe and free to behave exactly as they wished (although, of course, so would everyone, not just people with SN).
however, since we all live in the real world, not some distant Utopia, then there are certain behaviours which are not acceptable in society. to be part of that society, people should try their hardest to meet the rules which that society prescribes.
and so it is down to everyone to moderate their behaviours as much as is possible, and for the families of those who have difficulty moderating their behaviours, to help them out.
I doubt this post would have been made if the family of the boy in question had been seen to be doing their best to mediate the situation. but they seemed not to. and so you can hardly blame the OP for feeling a bit put out. if the family in question had adhered to the general rules of society (which, remember, include sitting in the seats which you bought tickets for), then none of this would have happened anyway.
I do know how difficult it can be taking a child with SN out and about. there have been some interesting viewpoints put forward as to why the father might have moved his ds to the free seats (although I don't accept that 'maybe the boy needed more space around him' as a valid one, tbh - how could the family count on there being free seats?), but to not have done anything at all to try to minimise the disruption for the OP is not fair. even an explanation from the family to the OP's family would have been better than doing nothing.
I don't agree that people with ASD should not have to moderate their behaviours at all. of course, some may not be able to, and that is a different matter, but given that the family were happy to leave their ds in a row by himself, then they clearly had some level of expectation (I would hope) that he knew some of the rules of society, and it would not be too much to ask that they helped their ds with the aspects of being at the show that he was struggling with.