Do they care about the "youtube" trademark used in other domains?
"PWN!" is a frequently used word on the Internet, the meaning of which clearly makes the words "PWN! YouTube" a parody.
"Mom can I go on pwntube to get a video for my homework?" "WHAT?!?! PORNTUBE!?!? NO!!!"
;)
Those who want their sites/products to go mainstream without ridicule (ExpertSexChange.com/ExpertsExchange.com is a popular example) or false negative impression/bias. This is why people often spend a lot of time to choose a name carefully, especially when you consider the long-term branding and marketing.