Horrible idea for a business, IMHO. Collision repair in particular can take VERY long periods of time with much of that time just waiting for parts or freed-up labor. It would be perfectly normal for a customer's car to not be touched for a day, even several days, while waiting for parts. Or even if the parts do get in, that work is done on some other cars to get them out first for a variety of reasons. All you'll get with cameras watching your work is a bunch of people constantly calling your up wondering why you aren't working on their car. Managers of those shops have enough of those calls to deal with as it is without people being able to question every business decision you make.
I just take my car to a reputable shop to begin with. And I don't take my car to any shop that won't let me walk around the shop. It's perfectly understandable why, for liability purposes, some places won't let you; I just won't take my car to any of them.
