It's appear on the liability side because it's the result 
of capital investment of a company and capital is a 
libiality for business (because of business entity 
concept). whatever capital invested by owner he want some 
return form it, thatwhy net profit become liability for 
business, it is so because business have to pay this(net 
profit) to the owner.

because capital is treated as liablities in accounts and 
the treatment of net loss or profit is either substracted 
or added in   capital for knowing the increment of capital 
or decrement of capital.

A bussiness/organisation/company is not a natural entity.
It runs on the capital provided by its 
members/owners/shareholders etc.So whatever is the profit 
is actuaaly of these persons.So its a liability.

Profit is regarded as the lifeblood of the company and also 
for the existence of business profit is the integral 
part,so whenever a business man investing money into the 
business he is always expecting returns for running his 
business smoothly .As both owner and business both are 
separate from each other that capital investment as well as 
return from this became the liability of the business.