Why is Dean's neckless called a Samulet? And not an Amulet?

because Sam gave it to him. It is like a symbol of their brotherly bond.

What fustercluck said.

But only some ファン refer to it as that, some - myself included - just refer to it as an amulet.

Samulet is just a nickname some ファン have 与えられた that amulet, because Sam originally gave it to Dean.

Because Sam gave him it.

Because sam gave it to him (when they were kids).