I just watched their demo video at the bottom of their blog post [1]. When making a purchase, they already show line items for Circle Fee and Bank Fee, they're just set to $0. My guess is that this is a promotion for growth, and at some point when they need to kick in a business model, these lines will stop showing $0.
Edit: Also, even if they are showing $0, they could be doing something sneaky like showing the price of bitcoin as higher than they are actually buying it for. If they are doing this, then you are actually paying a fee, it's just hidden.