They probably didn't want people to get a -10% penalty at -1. It's likely that they tested only changing the percentage and didn't like the result.
But -10% at -2 without the free amenity is literally the same as -10% at -1 with the free amenity. Whatever testing the developers did, the set of thresholds they settled on could be presented, with equal accuracy, in two different ways:
A. ) 1 Free Amenity per city (unchanged)
Negative Thresholds at -1, -3, -5, -7 (unchanged)
Positive Thresholds at +4 and +6
B.) No Free Amenity
Negative Thresholds at -2, -4, -6, -8
Positive Thresholds at +3 and +5
These two options are functionally identical. A city with the same number of added amenities will always be at the same happiness level under either system. Any change to happiness effects will apply equally to either system. The only difference is that Option B unnecessarily changes both happiness thresholds and free amenities, in opposing directions, to arrive at exactly the same result as Option A. This creates confusion, breaks mods, and results in a less intuitive starting point for negative modifiers.
This alone would be more than sufficient to seriously annoy my pendantic self. What makes it even worse, though, is the feeling that the developers pulled a bait and switch (albeit probably a careless one rather than a malicious one). Based on the livestream yesterday, any reasonable observer would have expected they were removing the free amenity but
not moving the thresholds in the opposing direction-
because that would completely defeat the purpose of the change. And this would have been a really positive change, making amenity management more of a challenge while also removing an artificial incentive to spam cities and avoid growing them. The fact the the devs were (apparently) making such a change was exciting. It gave me confidence in the remaining parts of the amenity rework. It made me hopeful that future patches would include similar important but low profile changes. Instead, it got turned around into a pair of contradictory changes that actively detract from the game while accomplishing absolutely nothing.