Although equally illegal, I would suggest there is a big difference between accidentally flying overweight, and not being aware of it, and with flying overweight, while knowing how much overweight you are, the position of the C of G, and the effect that this will have on aircraft performance, stability and handling
An interesting suggestion. Without having taken it through a test flight schedule in that configuration how would you know "the effect that this will have on aircraft performance"? The W&B diagrams weren't drawn up by someone who thought "Right, weight x, C of G at y, that's going to cause it to do z." They were arrived at as a result of flight testing.