I’m inclined to agree with the manager. My son manages a large CVS
pharmacy, and shoplifting is a daily occurrence in his store. Prudence would
dictate that a shopper, bringing in a knapsack or shopping bag from another
store, would leave it behind the counter. You can always transfer the
purchased items to the knapsack after checkout. Most stores do have a policy
of checking bags from outside for good reason. At another store where my
son was working as a checker on his first job, a man with a knapsack pulled
out a gun, walked up to him and told him to open the register and put all the
money in the sack. He didn’t move fast enough and the thief fired a shot. I
think he'll always now have a visceral reaction when someone enters the store
with a knapsack.

If you feel you’re being accused of theft, don’t argue with them. Insist the
police be called to document the incident, sue the store if you were
wrongfully accused, and buy the bike of your dreams.