There are syns in canned fruits as the fruit is packed into the cans with juice. Then in order to preserve the food and prevent any fermentation (which is when a can 'blows') the cans are sealed and pass through a steam cooker/pasteuriser.
So there are syns in canned fruits for two reasons:
-the syn value takes into account the juice and this has syns in just as normal juice has (unless you are using a value that is a drained weight-I don't have a book to hand)
-during the canning process the food is cooked, so as with all cooked fruit there are syns
In my honest opinion if it stops you being starving and picking the less healthy version (for me a bar of chocolate) it's still a good plan
Hope this helps!