PIA Flight from Lahore to Karachi ends up landing in Muscat; Here's why!

Lahore to Karachi flight lands in Muscat
Leaving relatives of passengers in a state of confusion, three flights of Pakistan International Airlines (PIA) scheduled to land in Karachi were diverted to other cities due to heavy fog on Friday morning.
Flights PK-214 from Dubai to Karachi and PK-307 from Lahore to Karachi were diverted to Muscat while PK-732 from Jeddah to Karachi was diverted to Lahore, PIA spokesman Danyal Gilani said in a statement.
"Due to heavy fog, the visibility was less than 50 meters, hence the plane could not land in Karachi,".
He said the two flights were diverted to Muscat as visibility at all airports near Karachi was poor.
The flight map on the plane tracking site FlightRadar24 shows PK-307 coming from Lahore circled over Karachi and then proceeded to Muscat when it was rerouted.
PIA flight lahore to karachi lands in muscat
PK307 Flight path (Source: FlightRadar24)
After the fog cleared, the plane took off for Karachi from Muscat and landed at 12:07pm.
"Now with improved visibility flight operation at Karachi has been restored," the statement said. 
One may wonder that why flight took off in the first place when there was fog at destination, and flight time is just one hour, as if they didn’t know of weather before takeoff.
Secondly, why not bring the jet back to Lahore instead of flying people without passports (since it was a domestic flight) to a foreign land?