For the same reason that British 'law' was not upheld at sieges either, because there was a general military assumption that in necessity military come before civilians (and arguably still is). It had long been custom in a seige that the military inside & out decided who got fed, who died, and when a surrender might take place and in which case who might be spared.
Sending out women & children from a besieged city who were then forced back under the city walls by the besiegers to die was not an entirely uncommon event, similarly it was accepted military practice that a city 'resisting' was open to any and all excesses on being taken. This regardless of the which side, if any, the misbegotten civilians were on.
You also have the old military practice of promising the soldiers a 'reward' such as the offering of Edinburgh for three days by an English General invading Scotland, despite the fact that Edinburgh was not in a state of siege nor even offering resistance.

Looking at Portugal in 1810 it can be viewed as one enormous siege, with Wellington taking the medieval approach of denuding the countryside around the besieged area - leaving the besiegers little choice of action, but to also revert to medieval practices, or else promptly retreat in disarray.
Had the siege somehow succeeded it would be understandable by military practices of the times that the entire remaining population of Portugal and the British army would have been open to slaughter, pillage, and rapine (or possibly rapine before slaughter, you never know with military types they had some strange customs).

Digging out various tracts on 'what a good soldier should do (in normal times)' seems a bit like hunting out all the various drill & tactics books published during the 17th & 18th Century, many of them fancy, grand and far reaching - but almost never put into actual usage because they didnt match the current military practicality or custom.
It can be interesting but not of much use in determining what should have gone on.

Until civilized nations agree on single combat by their political leaders to settle matters then war will remain uncivilized and uncivilized things will happen to civilians disproportionally to the military.

Jeff Lewis