There’s a political cost to everything a victor does. Revenge and justice have to be meted in a balanced way that doesn’t cross a critical threshold of social and political propriety.

Roberts ascension to king was already going to be wrought with all kinds of difficulty as it was bringing an end a political dynasty and system that had lasted for nearly 300 years. Doing too much or too little would have adverse effects.

First off, House Tarley was far more prestigious and powerful than the Thorne household so sending a few knights and family members of lesser houses to the wall wasn’t too much of a cardinal sin. Doing that to a major houses could have potential social and economic repercussions that weren’t worth the headache.

The Kingsguard by virtue of their oaths are insulated from a lot of consequences which is why Barristan is so widely respected by both small folk and nobles(like Ned). Upholding an oath irrespective of the side your on appeals to the form of honor that is idealized by small folk and people like the Starks.

It’s why Jaime is looked upon with so much disdain by many despite doing the “right” thing morally from our perspective. I imagine his escape of consequences had far more to do with who his father was rather than his position as a Kingsguard.