It's a nod towards convenience, and that's something Microsoft needs in its Windows 8x replacement.
Security firm AVG, makers of very good anti-virus software, both free and paid, see the inherent problems (opens in new tab)with this, and there are several.
First, the company points out "It goes without saying, that automatically connecting to open Wi-Fi networks is a bad idea. As we’ve explained several times before, not all free or open Wi-Fi networks are secure and others can be deliberately malicious". Yes, that does go without saying, or should.
Of course, automatically accepting terms for the user isn't such a hot idea either. There's a rather obvious opportunity for a man-in-the-middle attack (opens in new tab). This type of intercept is "an attack where the attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other".
This all can be disabled pretty easilly. Simply go into Settings, then Wi-Fi and then Wi-Fi Sense and turn it off. This is what AVG recommends users do.