That's true but we can expect a fair few this year, especially when the likes of Citeh are in town. We've seen all the threads about the silly money being charged for tickets on various resale sites.
Simple. Tourists. Football enthusiasts who don't support either club but have just come along to see Premier League football. Tons of people do this. Working at Heathrow I see loads of people coming through every weekend just to catch a game. Lots of people from Switzerland, Holland...