[giantfrog]

Because Firefox users have been clamoring for the ability to turn them off rather than the opposite.

[SahAssar]

I think you misunderstand. Firefox users have wanted this to be opt-in or explicit-choice rather than opt-out.

The implication is that all future AI features will be opt-out.

[ysavir]

I think the parent comment is snark. They're saying that since many Firefox users are saying "Let me turn off AI features, please!" for features they don't want at all, and few to no Firefox users are saying "Let me turn on AI features!" because few to no Firefox users want AI features in the first place, Mozilla is making AI features opt-out to "satisfy" the "want" of turning off AI features.

[LoganDark]

I think they're asking why it has to be opt-out rather than opt-in.

[ryandrake]

The likely answer is an incentive structure that rewards someone for maximizing 'number of users using AI'.