They're an ECN (Electronic Communication Network). Instead of making money on the spread, they make their money by charging a commission per transaction. Their spreads are so low because they reference a massive number of entities to find the absolute best spreads to operate under. So it's uncommon to sometimes find spreads as low as 0.0. Of course, very favorable for scalpers depending on their commission rates.
I actually preferred using a Market Maker to scalp on. I traded with MahiFX for awhile. They have tighter spreads but no commission which was better to me. For example, during peak time on a broker like OANDA I would get 1.2 on EURUSD. On MahiFX I typically got 0.8 with no commissions. Their spread is tighter because they act as their own market maker instead of passing along the transaction to another entity to foot the risk (and add another business that needs to get paid).