TVs intended for digital signage usually contain modular computers meant to be controlled by the user.

You seem to be downvoted by ad haters - ads being a sensitive topic - but in the context of a device made specifically for delivering ads, I agree it seems like a good idea.

In the context of being a reasonable human going about my business - I wouldn't want one anywhere near me.

Why would a business purchase a digital signage display that forced them to show someone else's ads?

I think they got downvoted because they misunderstood the idea. "mandatory" analytics and advertising would be working for the manufacturer, not the owner. To be an (antisocial) value-add it would have to work for the owner.