Of course they aren't canon. Even GerIta isn't technically canon and they're 'almost' the main pair.
That's the beauty of Hetalia. USUK isn't canon, but neither is FrUK, RusAme, IggyPan, or anything like that.
Heck, I'm a USUK shipper, but I accept that it's not canon, and I'm glad it isn't too. I wouldn't want to upset other fans if their ship was something else. The best part about Hetalia is that everyone can be happy, because there is no canon. (The only two-sided canon is HRE and Italy, and that's in the past)
But seriously guys, not every USUK shipper is crazy. I'm not, I try to be perfectly reasonable.
Actually it does.
In one of the Allied Meetings (I think the first) in the English dub by Funimation, when America and England start fighting France tells them to 'stop releasing sexual tension and get back to the meeting'. (Of course it's the dub, but that still counts...right?)
But in Paint it White the computer also says things about how France and England have sexual tension. (I can't remember the exact line.)
So pretty much, it's Hetalia- and everyone is shipped with everyone. So everyone can be happy and ship what they want! (You just have to avoid the crazy fangirls)
(I hate posting on comments that are months old. Sorry T_T)