I'm not sure if that specific type of frog is poisonous to garters (as some amphibians are), but as a general rule, most people advise against using any amphibians just for the reason that they typically contain a lot of parasites, that may be transferred to your garter.

I would not be shocked to find out that some garters occasionally eat some insects in some settings (wild or otherwise), even though it may be bad for them. What was suggested above is probably true. When something that moves is dropped into the tank, they are probably automatically thinking "food", without realizing what it is that is being dropped in. I certainly don't question that garters may take them, but I know it certainly isn't the best item for them to eat.