It's a sequence of problems that lead to this:

• The kernel driver should have parsed the update, or at a minimum it should have validated a signature, before trying to load it.
• There should not have been a mechanism to bypass Microsoft's certification.
• Microsoft should never have certified and signed a kernel driver that loads code without any kind signature verification, probably not at all.

Many people say Microsoft are not at fault here, but I believe they share the blame, they are responsible when they actually certify the kernel drivers that get shipped to customers.