You are making something out of nothing. It is not required to have a good reason to ban someone, as long as the ban doesn't fall into the "banned you, but really had nothing to do with you" category. It is much less required to investigate or provide any kind of proof for banning someone.
Besides, if someone is staying in the capital, despite having been cut his income, ain't many reasons for him to bother continue doing that other than spying. Why even bother with an investigation? Perfectly legitimate, regardless of how you may or may not like the reasoning used and the logic behind it.
