The summer start time is probably the best thing the PLL has going for it for a TV viewing audience, since there is little competition from other sports this time of year. A lot of people who would not otherwise watch lacrosse watch it due to all the other major sports besides baseball and WNBA being out of season.
But the summer also kills the potential for a larger live audience for the games. Families are always taking their big vacations, going to camps, or spending time outdoors/at the beach this time of year. The heat in any even remotely southern location can be a real turn off for a full day of sitting in sunny bleachers.
Pros and cons for the summer start, but considering a spring start would force the choice between NLL and PLL, as well as limit the impact of incoming rookies fresh off their college season since they would only start playing well into the schedule, I would not be a fan. Those are big problems, along with your average joe choosing to watch NBA and hockey playoffs instead.