Well, if you collect tadpoles, rear them to froglets, and let most of them go while keeping one or two you've actually helped the population rather than harmed it. (Not that it makes more than a minuscule difference to begin with. For all you know, approaching the pond to collect a few tadpoles scares away some heron or raccoon that would've eaten dozens.)

Taking an adult frog (especially a female who lays thousands of eggs) from the wild is much more harmful than taking tadpoles, and your chances of finding a captive bred specimen are slim.