I can offer an educated guess. Two reasons that you rarely see a mint tea that is based on wintergreen:
1. Nursing and pregnant should avoid its use (estrogenic effects)
2. Those allergic to salicylates (aspirin pain relievers) must avoid it.http://www.localharvest.org/mint-lovers ... -tea-C5469
For these reasons, most tea companies may be careful about using only very small amounts of natural spearmint oil flavorings or dried leaf in their herbal teas.