There wouldn’t be guerilla activity there anyway. The local population have no appetite for such things. The vast majority identify as Russian and have very little love for the Kiev regime that has been bombing them for years.

So far the main way Ukraine has been able to motivate people inside Russia to commit terrorist attacks has been through either bribery or blackmail/extortion. You can get a few individual terrorists with those methods, but you’re not building entire guerilla cells and underground resistance that way. People need to have actual ideological motivation for that. There are way more pro-Russian underground cells operating in Kiev controlled territory than there are pro-Ukrainian ones in the new Russian territories.

Russia just understands that infrastructure investment is an economic and social benefit not just to the region but to the whole country. Plus it makes logistics for the war effort easier.