Logic behind unsharp filter in matlab
The Laplacian filter does actually outputs values of 0 on edges, and positive/negative inside image regions. Edges are detected as zero-crossings of the Laplacian. 
This makes sense if you remember that the Laplacian is a second-order derivative, and not a first order.

So, the unsharp filter that you describe computes (identity - Laplacian) convoluted with the image, i.e., the image minus something that is zero on edges. Hence, high frequencies (edges) are not attenuated while lower frequencies are weakened.