Trello is owned by Atlassian. Not saying it's impossible, but they'd be shooting themselves in the foot big time if they pulled some shady shit that alienated their corporate user base.
I'd imagine they offer Trello for free to entice you to start paying for Jira (or god forbid, Confluence).
Leaking user data (or "anonymising" it and then selling it) doesn't alienate a corporate userbase, because corporations broadly do not care at all that user data gets leaked. Unless it materially affects their income, this is something that is ignored.
I work for a GMP manufacturer of pharmaceuticals and they use the free version of Trello and put tons of proprietary client information in Trello. Each client is a different company and they are never supposed to see data from the other one. It’s an absolutely insane thing to do in my opinion. I’m beginning to see why we are losing clients.
I'd imagine they offer Trello for free to entice you to start paying for Jira (or god forbid, Confluence).