The revealing isn't an automatic thing, actually, it's the area script executing an Explore command the first time you enter the area. The main Athkatla maps in BG2 (and a few others, like the conquered Keep or cleansed Temple) all have that code, but the Baldur's Gate city maps don't.
If a BGT or Tutu install is revealing the city maps automatically, it's because the area scripts have been modified. My Tutu install doesn't have those lines in the area scripts, so my guess is it's a BGT thing. If you want to remove that behavior, you can search for a mod, or you can do it yourself with Near Infinity. Open up the .ARE file in question, open up the area script that's linked there, and look for something like this:
That's how the BG2 area-reveal code is formatted, so presumably the BGT code would be similar. Just comment out that section by adding two slashes ( // ) in front of all those lines. That will prevent future areas from revealing themselves, but it won't un-reveal anything for areas already entered.