By this definition, any website having google adsense or other advertisements (Cj.com, Adbrite, all sorts of ad) are not permitted when using Yahoo API. This really puts a severe limitation if you want to be creative with yahoo api and make some money from your website. So basically... most websites using yahoo API are for hobby (without any ad)?
They'll catch most though... and the ones producing substantially... yet, breaching their TOS will get auto caught..