I'm afraid that's what they do by the time they get to mid July - its the reason I stopped buying them, I now just use surfinia trialing petunias in baskets. They make dwarf varieties of petunia now, they're probably better.

I have some of those dwarf trailing petunias and they're doing well, i bought them late in the season but i'm pleased with them and will definately buy them again.
No legginess !

Our growing season is long enough that we usually prune them back and give them a high nitrogen feeding about midseason--usually early February for us. Within a couple of weeks, they are starting to bloom again.