Looks interesting, but if your music flops you could spend a lot of money getting nothing back.
Yes, you are right but this service is for getting your track in lots of different music distribution shops which saves you lots of time. The promotional part has to be done by the artist or by his management and promotion company.
The bands and artists that have suceed with Tunecore are the ones that have worked hard in the promotion and they have build an strong fan base. These fans then go to these digital shops and buy their music.