If the user snaps an object to the border of the layout, he expects it to snap to it, not be cut in half with 50-75% of it invisible.
Not really. Snaps do not work with object shapes but with objects Hotspot positions
If you were to limit snaps just to the border then you would never placed an object with centered hotspot next to the edge of layout.
Grids are not limited to the layout as well, they are visible only on the layout area but they extend over it.
This is not a typical image editing software where area of work is all what you get. In C2 layout size is just a visual representation of the game screen where you can place objects. Notice that you can place them not only inside the layout but outside too.
And if you turn on Unbounded scrolling then you will see that layout have no limits in size anymore - can have probably infinite size.