They are forcing them to fly the planes.
The company has already decided the value of the slots exceed the lost passenger revenue.
The company could not fly the plane, but the governments rules force the situation.
The rules were changed during covid to alleviate this, but now that the rules have changed back, we see the same situation again.
Imagine if you were faced with an equivalent dilemma:
You are in a situation where you will lose the right to park in your own garage for a year.
To keep your garage rights all you have to do is drive around aimlessly for an hour.
Do you decide the environment is more important, or do you want to park in your garage?