It's like with Cloudflare. The free Tier is what gets small companies and hobby developers in. And as they know your system but not the one of others, they'll recommend it to use when your company grows or their employer looks for an analytics system.

But I don't think it's predatory. It clearly worked for cloudflare and seems to work for Tailscale (they openly said they're using the same strategy). It would be predatory if others couldn't match that, but I'd argue many competitors could offer free plans for small websites if they wanted to.