"Free" doesn't like privacy
The “free” business model means a company must use data to make a profit. So for encrypted messaging systems, they simply must leverage the unencrypted parts: when you write your messages, who do you send them to, where do you send them from… That’s called “metadata”.
We also know cases of companies designing their messaging app so that it allows insiders to take a peek. Even if such a company is ethical and forbids itself from looking at user data, one can always subpoena or hack its way to the precious info.
So we agree: “free” does not like data privacy. Unless a messaging system relies on donations, the money comes from your data. Or your metadata.

But Seeld is free !
And I’m aware I just said that “free” is bad ! So what gives ? Well, you first have to realize what Seeld is.
Seeld is a small application. It uses a couple of servers at the time of writing this. That means: it doesn’t cost me much to run. You can only exchange messages for the moment. No space-hungry attachments, which would require buying more space and more servers.
Also, there is no SLA, no one to make sure that servers are running 100% of the time, like Facebook or Amazon. It’s on a “best effort” basis. Of course, I keep an eye on the servers every morning. I monitor the amount of subscribed persons and exchanged messages (only amounts; don’t forget I can’t see any details!). I can add a server of two if needed. But I sure hope I won’t get a million new users in a weekend!
That’s why Seeld is free: it sure works, but it’s not managed by a big company. A very small bunch of volunteers takes care of it.