Can Mint do this???

I have just spoken to Mint today to find out why they have charged interest on purchases when all I did was take out a balance transfer. They've told me that it is the interest from the 3% handling fee they have charged for the BT and put it under the description of purchases.

I questioned why I couldn't pay off my handling fee immediately to stop any interest occurring but was told it was not possibility to do this. Is this legal? If so I would strongly question the ethics of Mint - I certainly haven't come across this with any other credit card company.....