There's nothing dirty or secret about this. The whole point of docker is to be lighter weight than VM, because you're sharing a kernel with the host. It's no surprise, then, that you can't run a container with an OS using a different kernel from the host.
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It's an amazing technology. It's a useful technology. It's still brittle and complicated and not a silver bullet. The impedance mismatch between the level of technical knowledge required to understand containers and their implementation and the availability and promotion of Docker as "just another app" will invariably create these problems.
Docker, the company, promotes Docker the product as easy to use. That's true for some definitions of "easy". It is true for fewer as Docker the product captures more mainstream users. To put it another way, the point of Docker, the product, is to be lighter weight than a VM. The complexities of sharing a kernel are implementation details. Ideally users would not encounter them unexpectedly. In fairness to Docker, the company, assuming comfort with technical complexity among BSD users is not a long reach.