iPad has mass media exposure that other tablets don't get - you see reporters and presenters using them on tv shows all the time so people recognise the product and know the name. TouchPad is close enough to iPad that people may not even recognize it as being a different product.
I'm basing this on experience with the rest of my very non technical family and I wouldn't be overly surprised if a lot of people saw TouchPad adverts and failed to realise it wasn't actually an advert for an iPad... from the few TouchPad adverts I've seen here in the UK they are very similarly styled to the iPad ads, and people may just see a 10" tablet with "Pad" in the name... same thing surely... They then walk into best buy or whereever and ask for "that tablet thing I saw on the tv... the something pad"...
I'm not trying to cast all non tech folks as stupid, but having spent years trying to do tech support / tech education for my extended family there are a lot of people out there who really can't tell the difference - I'm related to at least 5 of them.