Well, one difference is no one buys frogs in a pet store that they intend to keep in an outdoor pond, at least that I know of. You can buy bullfrog tadpoles here, but good luck getting those to stick around once they are full-on frog. For one thing they are very territorial hunters, so something has to give if you have more than one. We've had lots of bullfrogs over the years but never more than one at a time. We've even noticed the smaller frogs are territorial in the pond - they all pick their spot and pretty much stick to it for hunting.
How far from home did you transport these frogs? There's no harm in bringing them (as long as they are native to your area - you don't want to bring in a non-native species). - I would just keep my expectations low about them sticking around.