Yes, outlook.com is a free email service and anyone can register for a free account (the same as gmail, AOL etc...). The spammers use automated bots to register thousands of accounts that are then used to send out spam. I'm sure Microsoft and the other providers are constantly removing scam accounts as and when they become aware of them but they can never win the battle as the scammers just set up more accounts as quickly as the old ones are are closed down.