Why atheism always has a burden of proof

[Rahul]

Vinny, Vinny, Vinny.

I suppose you don't believe in Mermaids. Therefore you must be making a positive claim by being an amermaidist according to your reasoning.

While amermaidism is defined as "a lack of belief in mermaids" and thus makes no positive claim, amermaidism itself does not escape positive claims. What positive claim?

There are several, and they are all implicitly entailed by amermaidism.
a) The claim that the burden of proof for the existence of Mermaids has not been met.
b) The claim that amermaidism is a more rational position than mermaidism.
(sometimes) c) The claim that mermaidism is irrational.

You cannot be an amermaidism without affirming (a) and (b), and sometimes ©.

So if you are an amermaidist, you must affirm (a) and (b), and since they are positive beliefs, they entail a burden of proof.

Therefore I ask you to defend your positive claim that Mermaids do not exist.

I await your response.