They see people buying prepaid phones and want to get in on that market. They decide they will sell cards that give you more time running your cheap little computer. The problem with this is that the mobile phone companies are selling you a cheap phone, then selling you the cell tower service in metered bits. Microsoft is trying to sell you the right to use something you already purchased, your computer, in metered bits. There’s no service they’re selling. This whole thing, at best, is a dis-service. ;)
Here’s hoping that the $100-$300 laptops they release are reverse-engineered and linux put on them in the first week.