The backstop (which Theresa previously agreed to but in now trying to go back on to appease Rice-Pudd's mob) is to protect the Republic of Ireland, and the borders of the EU.
The Republic of Ireland is a member of the EU.
The EU is committed to protecting the interests of its members.
The EU has always said that a backstop is necessary to protect the interests of its members, in the event that nothing better can be agreed.
This is no surprise to anybody.
The only person trying to change their mind and go back on their word is Theresa.