Thanks for the feedback everyone.
Personally I have no problems whatsoever that the facility could serve as a spaceport (after all, there is plenty of space) but fact is that the structure is referred to as "Earth Spacedock
" and not "Earth Spaceport
" which would be a much more appropriate term if that was the main function, not to mention that this would help immensely to differentiate from the spacedocks we had seen in the previous movies.
Add to this Captain Styles' astonishment "How can you have a yellow alert in spacedock?"
If the facility would also allow the arrival of non-Starfleet vessels I'm sure there would always be a chance for something happening that's outside Starfleet's control and could be reason for a yellow alert.
And according to ST VI a "dockmaster" is in charge of the facility, not a "portmaster".