I will keep this rather short and simple.

Its all about what the cam's exhaust profile is set up for.

Many cam profiles use the momentum of the flow of the exhaust through the header to help scavenge gasses from the cylinder.

When you dramatically change back pressure you also change the flow characteristics of the gas flow through the header which can completely disrupt the flow dynamics the cam is designed for.

There is nothing wrong with very low back pressure as long as you have a cam and the proper header geometry to take advantage of it.

The problem is that most streetable cams are not set up for the flow dynamics of a straight header opening to the atmosphere.