It is their right to ask her to leave, and her being pregnant has absolutely nothing to do with it. Her pregnancy is only being mentioned to garner sympathy for her.
Her being pregnant did, however, have everything to do with the insulting comment of 'get scrabbled, oh, you already have been.'
Suppose this were an 80 year old customer, who, on the way out, felt tired and wanted to sit a minute before walking on? Or perhaps if it were a diabetic customer who had stopped in to buy a drink because their blood glucose was dropping, and realized as they were leaving that it was dropping too quickly for them to safely walk on sipping the drink, as they'd planned? I'd think that the store would do better to instruct waiters to say, 'Are you feeling all right? We noticed that you bought a to-go cup, but didn't leave.'
Personally, I would have thought that the additional 90 cents was to cover the cost of a waiter coming to me to take my order and serve me my drink, and the costs of the busboy and the washing of the cup. It wouldn't necessarily occur to me that if I went indoors to order the drink, and carried it away myself, that I was being a special snowflake for sitting in a non-busy outdoor seating area. No more than it occurs to me that in a restaurant when 2/3rds of the table are empty, that I'm ripping them off by staying at my table, sipping my drink as I read to the end of the chapter.