We're not just talking about jails though. The OP was discussing systemd + containers. Switching to FreeBSD to escape systemd isn't that weird of an idea since most of the same Linux software will also run on FreeBSD. In fact I'm seriously considering switching my Debian 7 (Wheezy) servers over to FreeBSD rather than upgrade to Debian 8 (Jessie) and have deal with systemd. Anecdotally I've read other people consider switching away from Linux as well
You might like systemd and Dockers. That's great. But that's your personal preference. You shouldn't be so surprised when other people might prefer to run the same software but on a different platform.
On a tangent note: I also have a bunch of existing systemd systems - RHEL servers, ArchLinux desktops, etc - that I'm very happy with and intend to keep running Linux. I make this point just to emphasise that I'm not anti-Linux nor a FreeBSD fanboy. Just someone who's platform agnostic.
> Sure, then can use both, but why?
Why not? This isn't a sports team where you're expected to only support one product. It's quite possible to use multiple different technologies based on whatever fits a specific purpose better.