Why do i have to pay to amend state when i only need to amend federal?

depends on the reason you are amending federal. 

may states begin their tax return with adjusted gross income, so if the reason you are amending federal changes AGI, then that could impact the state return as well 

Not sure what you mean ... there is no extra fee to amend the return UNLESS you have to upgrade the online program to add the change.  And no one is forcing you to file the state amendment but if it did change with you amended the federal you SHOULD file the state amendment as well before they get the news from the IRS.