Because it uses the turn system, I think. force initiating combat forces the game to... well, combat, in the middle of the turn.
Even if you are not in combat, you are technically in a turn. You can actually experience this better in the Tchort's eye area(+1 debuff ticks per turn), but here is bare bones example.
1. Turn starts. You get 50AP!
2. You open door, you spend - say 20 AP to open a door. You now have 30AP!
3. Initiate combat.
4. You still have 30AP!
5. shooty shooty bang bang etc.
6. Turn ends.
(this also means that any action other than moving potentially cuts your combat initiation AP.)
This can be migrated by waiting few seconds before action starts.(I think 1 turn consists as 6 seconds, so waiting 6 seconds before going all out is pretty good idea)
However I still want the system to be fluid, and therefore want stealth combat starting AP to be 50AP.