How can Apple tell that the liquid damage in my mac air was made by me?

Apple said my five months Mac Air has liquid damage inside and I need to pay more than $400.

I totally cannot accept it.

Please have a look below two pictures from Apple.

I can see corrosion, but how can Apple tell this damage was made by me?

Actually I can easily approve this was definitely not made by me because of this part's location.

Please have a look below picture. I circle it by black on the left above corner.

This Part is facing to the ground and is covered by the case like below picture.

All of above picture can easily approve I　cannot put water inside to damage this part without damaging other parts.

1. if water from keyboard, water will definitely pass by other places and damage other parts.

2. if water from buttom, there is protected by the case. How can I put water inside?

I believe that my case was just Hastily delt by one junior staff and later the manager just tried to cover his staff's mistake.

I know the apple's product is very good. But if apple treats customer like me, I believe sooner or later Steve Jobs would cry in heaven.

One of my friend told me normally there are several paper inside laptop and they will change the color if touching the water.

So people can tell the laptop was poured in water if several paper change the color togother.

I think this way is fair enough. But in Apple store when I asked the manager whether Mac Air has this kind of paper inside.

His answer is "not sure". I really do not know How can this man be manager!!

can somebody tell me whether mac air have this kind of paper inside the body?

Thanks,