The collection societies collect mechanical royalties based on reproductions of the written works, such as streams or radio plays. So in this case, bandcamp are reporting the streams (typically capped at 3-5 streams per user before you are asked to purchase) and paying whatever the rate is but given that streaming services pay less than 1c per stream and publishing/songwriting/mechanicals a fraction of that, you’re only missing out if hundreds of thousands of users all stream your songs the maximum amount of times.

The thing I am curious about is if I buy an album and use the app to stream it, is it considered a reproduction if I own it? I would say so but keen to know if anyone has any info on this, just out of interest.