I too am confused and rather frustrated in this regard.
I currently use Foundry VTT for my virtual games, and I also have IRL games. I own the PDF's and there is just no way for me to get a hold of these maps. Normally you would (at the very least) be able to get a high-res version of a battle map with a seperate PDF containing all the maps, that is not the case here.
I do not understand it and it is so incredibly infuriating not being able to get a hold of it. It'd be one thing I would complain about if I had to purchase maps seperately for adventure paths to be able to print them, but the fact that I am completely (and utterly) unable to get a high res version of ANY of the maps for Attack of the swarm 3, makes me just not want to be bothered anymore.
I know this might sound like an overreaction, but when I buy a pre-written adventure, the last thing I want to have to do is to re-make all battlemaps in dungeondraft (for example.) And since these maps DO EXIST IN HIGH RES in (for example) fantasy grounds, I just do not get it. I do not understand!