@sokrati the existence of the WiFi only targeting makes this a little confusing, but let me see if I can clear it up.
Continuing your example: when you target the carrier ‘Verizon’, you’re simply targeting users of Verizon and how they’re connected at the moment the ad is actually served is completely irrelevant. Additionally, you can do things like only target users who’ve activated with Verizon in the last X amount of time. All of the carrier targeting criteria works in this way and how the user is connected at the time the ad is served does not matter with the exception of WiFi only.
Now let’s say you built an ad for a large mobile game (20mb+) that you want users to download and install. If you target users who are on cellular connections when the ad is served, they may not be able to actually convert and download the app over-the-air. In this scenario, you’d want to add WiFi only to your targeting criteria so that you can also narrow your audience down by current connection type.