When you go to a business and portray yourself as a customer, you are sometimes offered certain enticements. For example, I did a shop at a gym and was given a free 3-day guest pass. Other times it might be something like drinks and snacks. My personal feeling is that, since there is zero chance I will actually be patronizing these businesses, it would be unethical to accept/use the hand-out. I just wanted to get other people's opinions on this.

You are posing as a typical customer. When I go to car showrooms, I sometimes accept a coffee when waiting to do a test drive. There is nothing wrong with this.

Since you are mystery shopping, you will have to portray yourself as a customer or potential customer. I understand where you are coming from, but at the same time, you have a job to do. Companies that pay mystery shoppers want to know how things are going at whatever place of business you are shopping. I would think that being offered drinks, food, etc. is to be expected and you could accept or refuse by saying you are fine.

Why is there zero chance of you becoming a customer?