It is because the canning of the fruit brings out the natural fruit sugars in the fruit itself..
I use a lot of tinned fruit in juice, but I always drain it and rinse in under cold water before I use it, that way I know I have washed off all of the sugar - the fruit still taste's as good, and of course I still syn it.
Also tinned fruit is partly cooked prior to canning. This makes it softer and easier to consume greater quantities than if you had eaten the fruit fresh. Any fruit juice has a syn and all tinned fruit juice is no different. x
I was eating huge tubs of fruit in fruit juices everyday for lunch and thought it would be free! iv been doing this 5 days a week for about 4 months and only found out last night that they should be syned. It totalled upto about 10syns EXTRA i was eating a day!!!!! ooooops