I'll take a stab at that one.
The 2nd Amendment clearly states a citizen's RIGHT to bear arms shall not be infringed.
There is (an albeit small) fee of $10 to obtain an IL FOID. Without the FOID, you are prohibited from purchasing or possessing firearms and ammunition.
Since when should we have to pay a tax, surcharge, or fee, to access a constitutionally granted RIGHT? Not a privilege, such as a driver's license, but a right.
Second, any purchaser of arms in Illinois, like any other state, already has to pass the NICS background check. So, not only is the FOID redundant, it amounts to a tax on a constitutionally guaranteed right.