People looking to catch ball pythons would be more likely to look in burrows, often during the day..... To increase chances of locating a mostly nocturnal species with minimum effort. So as far as those people are concerned BP's only live in holes in the ground and termite mounds etc.
Gathering ball pythons is not necessarily an indication of any indepth understanding of their behaviour, just understanding of a small part of it.
Saying they won't climb except for thermo-regulation is like claiming they "periscope" because their heads are cold/hot.
That these animals use elevation to examine surroundings or obtain food should be no surprise.
With their body type I doubt anyone is making a claim that balls are even near semi-arboreal, just that the can and will climb for reasons which may escape us.