I figured they were taking advertising money to fund both their work on the the app itself (which is free) and for maintaining the server infrastructure that allows them to stream quickly (which can't be cheap to them).
Not to mention that stitcher removes any editorial control over the kind of advertising associated with any given program.
Edit to reply to below: that has been the case for the download model. How certain are you that this is still the case for the streaming model of this app? Have you sniffed the traffic?