I think those agreements come from the fact that the agencies cannot 'spy' inside their home country, but can get information from other agencies that happen to

I think those agreements come from the fact that the agencies cannot 'spy' inside their home country, but can get information from other agencies that happen to have info about their country. I still think it is illegal for those other agencies to spy within the home country. Not to mention countries they don't have agreements with. The home agency does not do anything illegal itself, but the other agency does illegal stuff for them.