How would you suggest calculating weekly TSS? It would be quite confusing if changing this setting would recalculate it.
Weekly TSS could be total TSS over any selectable number of days, so 7 days for weekly if that’s your preference, or any other number of days like 5 day TSS, 10 day TSS, 12 day TSS…
The 7 day week starting on Mon would still be the default, but it would be pretty easy IMO to enable those who need something different to fit their schedule.
This is really all that matters. Similar sort of thing with heathens who can’t understand how dates are properly formatted! 
When you say ‘weekend’ - do you really mean just Saturday?
Of course not - you mean Saturday and Sunday. Ergo, the end of the week.
People from the US really need to get out more. And I say that as someone from the US.