I see American presidents sign treaties with lots of pens, its a tradition I have observed , what is this.?

There is usually one pen for every time a signature is put to paper. These become gifts to the other signatories, one is usually kept for his presidential library, and if it's a major treaty one might go to the Smithsonian. Basically, it's a way of giving away "the pen that President X used to sign the ____ Treaty", because each one was used in signing the treaty, so everyone receiving one gets an authentic one.

But it's not a uniquely American tradition. We stole it from those most amazing of all trinket savers, the British.

Pens used to sign important documents are usually given as souvenirs to diplomats and others. Presidents now sign with multiple pens so that more of these souvenir pens can be distributed, each the "authentic" pen used in signing.

I once saw an Indian advert in Saudi Arabia: the famous Senator pen of the 1950s has been recreated as the last word in pretentiousness! LOL

Common since. Even from the very early on they signed with what used to be pens, because if they signed with pencils, it'd be erased, etc, ever think of that? Besides, pens were invented and more commen then pencils.

They are given away as souvenirs.