SON OF A BITCH MUST PAY
Does it mean, when my turn comes I can perform one less action? So I can't take advatage of this mechanic, and do free fights by standing there and pray that the right enemy type activates?
Yes you could just stand in one place and wait for the enemy to get activated and attack you.
Another thing is the "aggressive" enemy ability. It says the fights triggered by it are NOT actions.
Does it confirm it, or just telling you, how to treat the "aggressive fights" when it happens during the player's turn?
Aggressive triggers whenever an enemy is within range of your character. So if you move within range during your turn you immediately have to fight the enemy because it attacks you. This does not cost you an action as it's the enemy attacking you.
Also Aggressive can trigger during the enemy activations because usually an enemy can either move or attack but not do both. However an enemy that moves within range during it's activation that is Aggressive can then attack you immediately after the movement.