Unlike other EC2 instances, you have to "reserve" a host, and the MAC instances require a minimum of a 24 hour reservation, that you pay for regardless of the instance state.

It can be hard to justify $15.60 every time I want to test some random thing. Not to mention I seem to recall you can't actually cancel a reservation till the 24 hour window is up, so you need to remember to stop it the next day, or automate that action somehow.