You may have a bad PCV valve or your rings are getting worn out
Your PCV valve controls pressure within the crankcase and assists in dispensing excess oil and running it through the engine as well as ensuring that an excess of internal pressure in the crankcase has no negative effect on your crank and transmission seals.
It is normal for the crankcase ventilation to put oil in your airbox IF you have a condition that would explain this.
Check the following items
PCV valve - ensure it's functioning properly. You have a reed valve type and the reeds should be in the closed position when not in use.
Rings - if your rings are bad you will be getting too much positive crankcase pressure from exhaust gasses blowing by the rings into the crankcase. You can test this using a leakdown test.
Too much oil - if you have too much oil in your crankcase it's very common to see that oil showing up in your airbox and fouling up your air cleaner. Make sure the bike is vertical when you check your oil level. If you have too much oil, drain a bit out so that it's at factory recommended levels.
If you require guidance on howto perform a leak down test, here are some instructions.