Spice Jet Airlines — duty free item, not allowed to carry

Address:

123

I bought an item in duty free shop, they gave a free briefcase. I asked the shop, whether they allow this extra bag. They told as a policy anything bought in duty free should be allowed. But, while boarding spicejet flight, they asked me to throw the bag and in fact threatened, me to pay over weight fee. This is first time, i come across this type of arrogance by an airline staff.