Blame Marriott and the point treatment of the Residence Inns. They've always been $5, even on high rates (Downtown Houtson RI $179 vs the Courtyard next door for $179, in the same building, yet 10 vs 5 pt per $). They're doing it to stay competitive.
As for why - Some people enjoy the extended stays. I'm currently @ a Staybridge Suites in MN and have to say I like it (and its a better property than the Candlewoods I've been in)