I think that while HNN is the most trusted source for speculation maps, they don't have a monopoly on them.
They have a collection of insiders that give them amazing input to develop the maps...that arguably prove accurate 40-60% from the beginning.
Other people might make SPECULATION maps that cater to other groups, if the FB page - that everybody hates - speculates that XYZ is coming to the event, someone making a map for what that group is speculating would be accurate in capturing the consensus.
Very few people have ties to actual people in the know within Universal, so if they use the HNN Spec Map as one of their biggest/most trusted sources, it make sense.
Universal doesn't call up HNN and Legacy, tell them the lineup, and say "have fun making up hints and clues for your peeps". It's a process for them to make inquiries to sources, try to connect the dots, and only when they feel confident they've "shown their work", to release it a spec map. And then it's only an average of 50% correct.
If people accept that nobody, outside of Universal's inner circle, knows the full truth and understand that a SPECULATION map is just that, a collection/amalgamation of speculation, and to know that anybody who puts together a map is some insider with full knowledge, everybody would have a lot more fun with speculation.