C-stock blinds down

I was waiting at Tower Hill earlier, and a C-stock drew in with both the central and the right-hand side cab window blinds fully drawn. While the train was in the platform, a D-stock approached from the opposite direction, and the driver pulled the left-hand blind down as well! Any ideas why a driver would want to black out his cab*? Is it even permitted to drive around like that? Presumably the first two wouldn't have an adverse effect on visibility...

* before anyone makes lewd suggestions, the "offside" door was also open, and it was obvious that the driver was the only person in the cab.