Mozilla wants Firefox to be a mainstream browser that anyone can use. Ad blocking introduces the risk of site breakage, and the majority of the population don't have the knowledge to deal with it.
In 2008. You know, the year the majority of Americans didn't approve of gay marriage? [1] The year Obama said that marriage is between a man and a woman? [2]
Applying modern sensibilities to history is stupid.
> It's probably the kind of user that has telemetry off.
If less than 4% of users use uBO, which the kind of users you're referencing claim is the primary reason they use Firefox, I doubt many users disable telemetry either.
I believe some Linux distributions patch Firefox to change default settings including disabling telemetry. Probably not a big factor, but still something to think about.
This is what Firefox blocks by default: https://support.mozilla.org/en-US/kb/enhanced-tracking-prote...