The "loan" distinction is important not for semantic reasons but practical one -- they are offering small businesses cash advances. These are different from loans because they are not secured. There are no assets put up as collateral. However Square believes they can do this better than other services because they can effectively secure the advance by taking a piece of each card swipe at the merchant's point of sale system. But if the business goes under, Square has no claim on any assets.
I don't totally understand why/when merchant cash advances work, but I believe that adequately captures the 'how'.