It's unlikely they can make a profit doing that. Servers, devs, etc cost money. This way they keep the audience smaller and can make money as well. It's a business, so you do what you gotta do.
That's arguably a good thing since a premium model mitigates the temptation of ad or data selling based approaches. But the fact is many don't care nor mind those as long as they aren't extremely intrusive. Even if they are (full page banner ad with small x), they may tolerate it anyway.