It seems to me that you can self-host. There is reasonable installation documentation at http://mortar-framework.org/. You just have to set up the infrastructure yourself, which you would have had to do anyway if they didn't offer hosting. It also seems that you can use their platform for only the cost of AWS if you don't mind open-sourcing your recommendation engine, which sounds terrific if your project is itself open source. Honestly, I can't find any qualms with this. Their business model seems to be that you will probably pay them $100/year (or $500/year for team access) because it's easier/cheaper than DIY.