During the Uber vs Didi era, they paid the customer 10-20 yuan for each ride. Occasional free bike ride is nothing I guess. Especially when you have a huge deposit that's very unlikely to be refunded. Also, you probably happened to use them during very competitive times (since last spring). I've used these bikes since last year, for most of the time, you do have to pay.

they pay you to use it. if you are surprised for the 0 RMB charge, think about those people riding those bikes to collection Hong Bo as their day jobs.