Do they have any artificial restrictions? Last time I tried a similar app (it's been a few years), I wasn't able to get an audiobook because someone else already had it "checked out".
I suspect it has something to do with licensing, but I don't know for sure.
You may want to live in a world where a publisher sells a twenty Euro book exactly once, but that's not sustainable.
If you want more concurrent users, pay for it. The library has decided which level of concurrent users to buy.