Once sovereign

Sovereignty is inalienable. When other rights are inalienable they derive that inalienability from the inalienability of the sovereign. 

A people cannot sell off to aliens their claim to their land because that would violate the inalienability of the sovereign. 

So long as the Jewish people were not sovereign in the Land of Israel it made sense for them to think they could sell off their private property to aliens during the sheviit year.  Once the Jewish people become sovereign, that option takes on a different meaning. 

If the Jewish people feel they can exercise heter makhira, then ipso facto they are saying they do not have a sovereign claim to the land. It is small wonder the Haredim are comfortable with heter makhira since in the first place they feel no particularly strong connection to the land, and even less so to a sovereign claim on the land.