This has always been the law for Illinois sales tax. A company with some physical presence in the state is responsible for collecting the tax on Illinois retail sales. Amazon dodged around it by using "affiliates". Amazon didn't collect, but technically, any Illinois affiliate shipping to an address inside the state was required to collect it.
There is also a "use tax" in Illinois. It has been in effect since '55. In effect if you buy items from out of state for use in Illinois, the purchaser is supposed remit the tax. (you get credit for sales taxes paid to other states). Reporting this has been on the honor system, and as a result nobody ever does it.
Amazon could have probably stayed ahead of the curve by only connecting interstate buyers and sellers. But it would not have lasted forever. The bottom line is that you owe tax on all purchased items used in Illinois. They have not chased after mail order and internet sales because the paperwork would be a nightmare.
The computer revolution is changing that.
Paul K. Dickman