Paying tax on free food items

I have never understood this so may be someone can shed a light. You see there are several food outlets that come up with promotion e.g. Buy 1 and get another at 50%!

But the thing is you have to pay full tax on that another item which is priced at 50%. In fact in many cases it is Buy 1 and Get 1 free but you have to pay full tax on that free item. What kind of logic is that? I mean the tax is paid on selling price and if my maths is right than (assuming GST is 17%)

0 X 17% = 0

So whatever the GST rate is, when something is multiplied by zero then end result is always zero!

Why do we have to pay full tax on items which are free?

gst is a scam, every business has a fix with tax man, gst collected is more like business owners extra profit, whoever think that gst collected on every single item is forwarded to govt must be smoking too much weed......