That’s because YouTube is on the whitelist of sites that TL2 people can link to / embed. Back when I created a thread asking for some extra domains to be added to the TL2 whitelist (such as d3planner, imgur, diablo3.com and so on) the thread instead got hijacked by people embedding dog and cat videos from YouTube into it. My reasoning was that if there were domains that would be frequently linked to (e.g. for build guides, screenshots, and so on) available at TL2 then almost no-one would care about obtaining / maintaining TL3 except for the people wanting to make guides / wikis on the forum.
Here it is…
It seems ludicrous to me that anyone and their uncle can embed YouTube videos into the forums but we can’t create a link to Blizzard’s own armoury site unless we have TL3.