Poor Replaceability
The Scout-bots aren't exactly cheap, all those rare-earth metals in their leg-servos and sensors and other expensive parts mean that they're not disposable for a civilisation operating in frontier conditions on Mars. They're built using equipment imported from Earth, which is a hell of a supply-chain.
So they're armed with a gun for self-defence, but their best defence is to never get into a fight in the first place.
Instead, they park up and provide data for the much more replaceable soldiers and robust ground-vehicles used by the main military forces.
You could feasibly build autonomous soldiers (perhaps they do back on earth!) but they'd be hard to replace if destroyed, and when it comes down to it, it's easier to show up with a dozen soldiers in a massive armoured tank.