Obviously there's a lot of demand, even if the (assumed) cost is not justified due to the small size and poor sun angle/exposure. People will buy it, it's not hard for tesla to put the PVs in the cars when they already have a glass roof, have a plant putting solar cells in another glass roof, the solar roof part probably would not affect their manufacturing at all compared to the glass roof.
A) It doesn't make sense to count the value of a solar roof as just the cost of energy it generates. If it generates 1-2 miles a day, and you drive your range total once a week, that is half the value of upgrading battery sizes $10,000 to go from an S75 to S90.
B) There is a small efficiency savings in going solar DC directly to the car's pack, instead of home AC conversion to car's DC and voltage.
C) there are going to be rare instances of extended airport parking, camping, driving out of supercharger range where it really helps. Especially airport parking - where you don't want to drive a long distance to an airport then leave it sitting with vampire drain.