Two reasons immediately come to mind:
First, as others have mentioned, there could be laws or mores in place to restrict this use. This is relatively simple to justify to a reader, but doesn't answer why they can't.
Second, touched on by a few answers: combat is horribly complex, and they really can't conduct combat reliably. Drones which can traverse difficult terrain, pathfind, coordinate search patterns, and identify things which might be of interest to human oversight are quite feasible even with near-modern technology. Autonomous maid robots which are capable of "tidying up a room" are presently a pipe dream - even with plenty of time to think. Effective combat is more complex (facing an adaptive and intelligent adversary) and requires rapid decisions.
So, just one of the simple problems in combat: vision. Vision must be interpreted and reacted on within split seconds - something humans are quite capable of. If you would like your scout drones to not fire on friendlies, they need to be able to operate at at least the human level. This is far beyond detecting things of potential interest. And of course, as another commentator mentioned, knowing when to stop hostilities...
As another answer noted: cost and resources. You decide to solve the vision thing by adding some really beefy computer processors to your drone, mobility with very high rotation speed joints, etc... Might as well add a bit more ammo while you're at it, some armor plating to protect the resource expenditure... Maybe a bigger gun... You've recreated a tank (or a Bolo). It's great at fighting, but what you really wanted was a semi-expendable autonomous scout unit which could detect and report back potential issues. You already have tanks.
And so, the design is purposefully limited such that you can send these out to identify where to send the tanks. Yes, you could probably use them to conduct minor sabotage via their point defense weapon, possibly at the risk of loss of both the scouting unit and the element of surprise, but that's not where their chief value lies. And in general combat they're a liability if they have even a relatively small chance of shooting friendlies, so they'd better lie down and not participate.
You could possibly use the scout drone or a similar relative with more carrying/dragging capacity to deliver a bomb, though that'd probably be too slow in many situations compared to sending out the tanks and troops with mortars.
TL;DR: Humans are extremely intelligent and adaptable relative to anything else we can find or build, and those qualities are essential in combat. Scout drones have their place, but general combat is not that place.
