Why isn't it the same? The fact that it is possible to change that default means google simply pays less for it than they otherwise would if it wasn't changeable.
It's not a binary toggle - firefox is selling you as a source of revenue for themselves. They're just not making it as extreme as it is possible to be - in the hopes that you don't switch away.
You can compare same situation with safari in iOS. Except google pays a lot more, since you cannot switch away in iOS as easily, and culturally there's more reluctance compared to firefox users. This makes google pay more for iOS traffic, as those users are worth more.
The problem is that this is equivocating between "selling your data" and setting Google as the default search. The former implies Firefox is harvesting your telemetry and personally identifying you and selling it off to the highest bidder. The latter is setting Google search as an optional default, where any telemetry is part of customary interactions with Google search rather than anything specific that Firefox is doing.
The sense in which you are the product on Firefox is that they want to maintain a large enough user base that search licensing is valuable enough to sell to Google.