It does not constitute as data usage because with Wi-Fi, you're using the internet connection from your house/coffee shop/etc. rather than the 3G connection.
Here's one way to see for yourself (or to allow only Wi-Fi mode): go to the Settings on your phone, and turn on Airplane mode. This will turn off 3G AND Wi-Fi. Then go to Wi-Fi and turn Wi-Fi back on. Now your phone-related features and data won't work, but Wi-Fi will still work.