Other than a potential ethical situation and customer relations nightmare, I think the biggest reason ISPs are against it is because it is expensive. Real time DPI of ALL of the data that passes through their routers would mean a lot of extra processing power. That is unless they don't mind slowing down speeds significantly. This is the reason why I suspect anything happens, only unencrypted traffic will be monitored. I don't even think it would be possible to decrypt, and even if they could, it would not be worth the effort.