My reaction to this is that they should just raise the damn fares. And I say that as a person who rarely checks a bag, especially not on domestic trips. (I have traveled for weeks using just a day pack. Though I do check bags for some overseas trips.) Nor do I fly American much, since their major routes of interest from here are to Latin America (via Miami), which is not one of my more frequent destinations. I do, however, figure that other airlines will start doing this too.
But I will also note that a lot of the people complaining didn't bother reading the announcement very carefully.
1) It applies to tickets purchased after 15 June. So, if you know you will be going somewhere in the next several months, you can just buy the ticket now and avoid the fee. It is not retroactive, despite anybody's fears that it will be.
2) It doesn't apply at all to international itineraries.
The concerns about people being forced to gate check a bag because others have oversize carry-ons are semi-legitimate. Only semi because American now has more incentive to enforce the rules on carry-ons. And one can always make sure to have a bag that will actually fit under the seat to ensure not having to gate check.
The question of code-shares is also simply answered. The rule is that of the operating airline. So, yeah, you do have to check a bit harder to see who is operating the flight, but you should do that anyway.
Overall, I think this was a bad move on American's part, but I think there has also been a lot of overreaction to it.