Russia wants to demilitarize Ukraine and change Ukraine government and  order to achieve that goal they need to have there occupation force that can keep status quo after their invading forces leave. It isn't easy to keep alternate government in hostile country, you can just ask how easy it was for USA in Afghanistan. Resistance doesn't need to be big in order to change governance as soon as  occupying forces leave. So in order to Putin to achieve his goal he needs to have total dominance over U