On the basis of the referendum that resulted in NWFO joining Pakistan as well as international law under which Pakistan is a successor State to the British and therefore inherits treaties and agreements made by the British, which includes the Durand Agreement on border demarcation with Afghanistan.
Under international law, the Durand Line is the legal and internationally recognized border between Afghanistan and Pakistan, which is why, despite all the whining and crying from Afghanistan, they have never dared take the issue to any international arbitration body because they know they'll lose.
That said, most people are sympathetic to the idea of open borders between Pakistan and Afghanistan, where trade & travel can occur freely. However, such open borders require peace and stability in Afghanistan as well as agreements on trade and uniformity on issues such as tarrifs and restricted markets, otherwise open borders just become a way for other countries to bypass Pakistani tarrifs or restrictions by being smuggled in from Afghanistan.
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