I'm not ultra familiar with the topic so if someone wants to correct me please do but :

- Linux has always been described as just a kernel, which translates as just a syscall table. The fact that this table is stable or not is not relevant here.

- *BSD on the other hand are shipping a kernel plus a lot of libraries/binaries, if you want to simulate a BSD system, you have to expose those libraries/binaries.

It's not so much a technical difference, it's more of a different approach to OS development (kernel space vs kernel/user space).