Why trademark(tm) these slogans?

Sure, you’re not going to see “Packed with peanuts, Microsoft really satisfies” but you could see “Packed with peanuts, Payday/Mr. Goodbar/Kung Pao Chicken/etc. really satisfies”. Trademarks protect valuable name recognition. Any slogan popular enough for people to know (like the Snickers slogan) is ripe to be ripped off.