Nah, that's just airliners abusing their monopolies to squees every last dime out of your pocket, the poor shareholders need to survive too, you know.
Its not just trans Atlantic either, it's everywhere. I just booked Mexico Canada and I got the luxury premium of being allowed to take one free carry-on
To be clear, 2°C is not going to significantly affect lift. Planes won't to falling put of the sky on sunny days. Rather, airports and weight limits were designed around historic temperature maximums, and much higher maximum temperatures are showing up much more commonly. Adjusting these limits isn't hard, but airlines are going to cry every step of the way and pass the cost ditectly to passengers.
Increased turbulence and higher winds are also a concern, increasing maintenance costs and travel times, as well as extreme weather shutting down airports more often.