And the spanish always won each battle right ?
Depends which Spanish you ask!
I have not seen any discussions on why Sam designed that rule but my own thoughts are that it again reflects the battles of the era.
Giving free entrenchments forces the (usually more numerous) Spanish onto the defensive which reflects the way they fought.
With the enemy in a fixed position as an attacker you either capture the position, or you don't. How can you "draw"?
In an open field encounter you can fight until both armies are exhausted and holding their start lines, thus a "draw". But if one side's plan is not to attack but to sit in its defences the attacker has to capture those defences to win.
Lasalle falls more into the "simulation" rather than "game" category (and is much the better for it). Why do you choose it over rules more suited to "balanced" pick up scenarios like Black Powder?