The majority of jobs are meant to be mystery shops. A client is paying to find out if their product or display is set up correctly. By letting the stores know what you are doing, the client doesn’t get a true view of what is going on in store.
It is not illegal to take pictures in stores. It might be against a store's policy, so they can ask you to leave. Also, no one can touch or take your personal property without your permission.
If you are approached by an employee while doing a job, it is really up to you what to do. You could tell them you are taking photos of a product to get a friend or family member’s opinion. Some Agents like to wear headphones while doing photo audits, so it looks like they are just listening to music. If they ask you to stop, then you should comply politely. If you are ever confronted by a store employee, please represent yourself and Field Agent well by being respectful and polite.
Some Agents prefer to not accept jobs that require photos because they do not want to risk being asked to leave. It doesn't bother other Agents. We recommend that you do what you are comfortable with while knowing that we never require any illegal action as a part of any job.