This has happened to me many times. I’ve never considered it at all demeaning.
Just because someone asks for help doesn’t mean that they think you work there. Most employees have badges or wear specific color clothes or vests. Sometimes people just need help and if they feel they can trust you, they will ask. You must be trustworthy.
I get asked for help all the time in Home Depot and Menards for some reason. It's funny because I'm typically the least-handy around the house guy I know! Seriously, changing a kitchen faucet is an all day job for me.
I certainly don't dress like a Home Depot or Menards worker either, meaning I don't have a store shirt on.