Thanks @Prostheta. I had asked this question in a different forum and was told:
"EMGs are grounded to actual ground the same as any other pickup, through the cable's ground. It's just that they're quiet enough and internally shielded well enough that you can get away with not grounding to the bridge."
You are saying virtual earth (contained within the pickup itself?) whereas the other answer refers to actual ground through the cable (from guitar output to amp input). Are both somehow describing the same thing in different ways? Or would you say the other answer is simply incorrect?
To clarify my original question:
Other manufacturers have released active pickups in the years since EMG first appeared. Yet, all other manufacturers have chosen to retain the traditional connection to the guitar bridge. So, I'm wondering what disadvantage (not necessarily monetary cost) they see in the engineering decision that EMG arrived at.