Can't modify disable Enforce user logon restriction at a Main domain controller however is possible to disable via a Member of of Domain Controller and why?

I configured a Member of Domain Controller and I went back to the Main Domain Controller and open gpedit.msc in cmd and try to disableEnforce user logon restriction radio button but is gray-out, however when open a gpedit.msc in aMember of Domain Controller I can disable the  Enforce user logon restrictionradio button. so why?

Regard,

Thanks.