Why wouldn’t you count it that way? If I get a job where I only have to work 75% of the year, my earning potential is salary + whatever I can make during the summer. If you can make the same pay/day, that’s a desirable 33% (not 25%) bonus.* The teachers in my family all take advantage of this, although the Bureau of Labor Statistics (PDF) tells me only ~50% of teachers work in July. Given that the average schoolday is only 6.7 hours (Nation Center for Education Statistics), one might think they’d also take on extra work in the afternoons, but they don’t feel the need. I can’t find information on how many of those 6.7 hours are taken up by non-teaching prep periods and lunch. My search yields one school district which guarantees an average of 3.25 hours/week of prep time each week. A reply to my email to family gives us a single anecdote of one prep period per day (actually more, but the extra time is usually devoted to meetings and such.) I visited her once and lunch was damn short.
The schoolday is short, and some may be critical of the average of only 5.6 hours worked per day, but keep in mind that this includes time spent working on weekends, so that adds up to an average workweek of 39.2 hours.
*An extreme example of this is a married pair of professors I know who clear an extra $98,000 each summer.