Great question that is why we put that answer in our FAQ:

How does ShadowBot not charge a fee?
ShadowBot is able to provide this service FREE of charge, through donations (both sbd and time). All funds generated through posts ShadowBot creates on steemit are used to fund development and promotions/prizes as well. Every person that has helped develop ShadowBot through code or other means has been a member and benefited and contributed back to the community! ShadowBot doesn't do what ShadowBot does for ShadowBot. ShadowBot does what ShadowBot does because ShadowBot is ShadowBot!
https://shadowbot.us/shadowbot/faq.php#how-does-shadowbot-not-charge-a-fee