there is no catch. their business model is not on repo count, but collaborators count. in other words, solo devs and very small teams can freeride, if you grow bigger you start paying.
BitBucket is run by Atlassian, their goal is to later upsell you to their line of products, so they loss lead with free unlimited private repos in the hope that if you need a project management/bugtracker system you might choose JIRA.
also, it's backed up by Atlassian.