The samurai get paid by the daimyos though right? So they get their metaphorical paychecks from the people who are getting more money. If the daimyos wanted to just cut their paychecks, they could get more money that way too. By fighting for their daimyo, they're keeping the money in their own pockets, so they definitely have benefits for working for their rights to having a decent dinner and some spending money when they get home.