One is that bacteria multiply faster in warmer temperatures. The other is because preparing food outside makes safe food handling more difficult.
Below are two sites that do a very good job explaining how to handle food preparation during the summer.
One thing the sites do not mention is to store your perishable groceries in a cooler with ice in it for the trip home from the store. Even a 20-minute trip home from the store in a hot car can potentially cause problems. Whole Foods provides complimentary bags of ice for that purpose.