In the free session after I got my M1 Macbook, I asked the same question to the apple executive. He said it's upto the customer if he/she wants to use a screen protector. If you don't want to use a screen protector like the people in his office, then you have to be careful while handling your device and periodically clean it with damp microfibre cloth. So there is no policy violation if you use one.
Personally I got a screen protector because I carry my Mac in my backpack and you never know how much pressure is there on the lid. Also my previous laptop screen has keyboard impressions on it. It's not a TV or desktop that you can always maintain a safe distance. So I applied a screen protector. The upside is I can change it whenever it is scratchy. The downside is increased reflectivity. The viewing quality is not absolute at at par with the naked screen but again the difference is marginally noticeable.
Cheers!!