I agree with you, I have done the same in the past (not this time). Because I arrived very early at the workshop, and papers were lying there unattended in the "Ineos" section I could see a workorder where the customer said clearly: "don't fill the washer fluid".What always amazes me they manage to squeeze in windscreen washer fluid, I have always been charged for it in dealer services for all my vehicles. I have always filled them prior to prevent there brand being put in (I swear by my Autoglym) and asked them how they managed to get it in, and why syphon mine out.
Next time I'll do the same.