That only works for the cases where the teachers are paid for that time in the summer.
Often, that is not the case, and instead they are working another job to replace the paycheck that stops coming during that period.
It's easy to blame the teachers for this, but I try not expect people to spend a quarter of the unpaid time I see teachers already spending doing class prep, let alone more.
(I'm sure that there are teachers that don't spend that time. I'm also sure that there are teachers, somewhere, that actually get paid for that time. But the ones I know personally already spend huge amounts of completely unpaid time on class prep, and often are just left out in the cold entirely during the summer unless they are teaching summer classes.)