it is the same reason why the F-16 have it and why it does not extend all the way.
A quick search on the interwebs and you'll find clear canopies and tinted canopies for the F-16... but when it's tinted, it's tinted all the way around. The small bit of the canopy that is separate from the one-piece that moves may or may not be tinted as well., but I don't think the pilot can see that far back anyway.
Still doesn't explain the purpose of the tint... or why it's front-only. There are sites on the internet that says it's for radiation protection in the F-16.... so I guess Mother Russia is short on cash and can only protect it's pilots from radiation from the front?