If I enter a store and they tell me I must leave my backpack in the front of the store, and my bag gets stolen, is the store responsible for my bag???
Yes.
Once you leave your bag with the store, at the store's request, a bailment has been created.
A bailment is defined as:
The temporary placement of control over, or possession of personal property by one person/entity, the bailor, into the hands of another, the bailee, for a designated purpose upon which the parties have agreed.
There are three types of bailments:
(1) for the benefit of the bailor and bailee
(2) for the sole benefit of the bailor
(3) for the sole benefit of the bailee.
You didn't request the bailment to be created.
Item 3 above applies in the situation you posit.
The store has an extraordinary duty of care, and would be required to reimburse you for all losses you could prove if the matter went to trial.
You are the bailor.
The store became the bailee, because the bailment was created at their request.
The bailee should inventory your bag.
If the bailee fails to inventory your bag, the bailor could claim three $5,000 diamond rings were in the bag.
Of course, the bailor (YOU) should be bale to prove he/she had purchased same.
The bailee demanded you submit your property to benefit him/her.
The bailee owes you a very high duty of care to protect your property.
You, on the other hand, owe the bailee NOTHING.
Bottom line, make sure you get a receipt indicating the store took possession of your property.
Better yet, don't patronize places where your property must be surrendered upon demand of the merchant.
There are millions of stores in the nation.
One need not submit to demands that demean or disrespect your humanity.
You can buy whatever it is you wish at hundreds of other, less intrusive, less demeaning merchants.
Even better, leave your valuables at home, or locked safely in your vehicle.
More information on bailments:
Definition of BAILMENT • Law Dictionary • TheLaw.com
bailment