This is unintended, or at the very worst, a happy coincidence for Apple. Consider the bullet-point version of this scenario:
- Apple's AHCI driver doesn't enable TRIM on non-Apple SSDs. Apple made a decision way back at the beginning of SSD support to not enable TRIM because of buggy firmware on early SSDs. They decided to eat a performance hit rather than have a crap drive eat user data. They likely have never revisited the issue, because those drives are still out there.
- 3rd party SSD manufacturers don't bother supplying OS X drivers that enable TRIM, favoring their customers to use a commonly available hack instead.
- commonly available hack modifies Apple's AHCI driver to do it's thing.
- Apple enabled driver signing in the latest OS to increase kernel-mode security. Something that everyone should be happy about.
- commonly available hack can no longer modify Apple's AHCI driver, because the signing would no longer be valid.
- 3rd party SSD manufacturers still don't bother supplying an OS X driver that enables TRIM.
- Apple gets hammered on Slashdot because they increased the security of ALL kexts, because one out of the over 200 kexts installed with Yosemite was being hacked by a 3rd party software to enable a feature on a 3rd party device. Shame on them!
The funny bit, is that you can turn off the driver signing requirement and hack the AHCI kext anyway, but OMG EVIL APPLE DOES EVIL!!! handwaving will drown that out.