This is also bad information. Kernel taint is a bitmap that represents a number of things:

  - Proprietary software (non-gpl-compatible, as you say. ex: vmware)
  - Crap (drivers/staging)
  - Out-of-tree modules (virtualbox)
  - whether the kernel has hit a BUG already
  - ...
  - lots of stuff

VirtualBox taints the kernel because it is out-of-tree, not because it is GPL-incompatible. Why is a GPL compatible kernel module(s) not merged yet?

Because the code quality is crap.