It's called buying them when they're on sale and (usually) saving money in the long run. Not to mention many people just get a bunch from Bing/Amazon promotions.
Hell, even if they only have like 300 or whatever (because they didn't use the entire amount of the points card because stored currency systems like MSP are fucking stupid), Microsoft should have absolutely no say in when they use that money. It's literally a way for Microsoft to steal from you. No 2 ways about it.
The MS store recently had a deal where you could get 1200 points for $5. Just because frugal customers took advantage of that deal, should they be screwed out of something they *purchased* (I'm aware that promotional points expire already, and am fine with that)?
Believe it or not, many of us don't have a "it's burning a hole in my pocket, so I better spend it" mentality with money.