A Japanese gastronomic faux pas

I pushed and jostled my way to the back corner of a sushi train restaurant in Asakusa, Japan. I found a seat adjacent to three very well dressed Japanese ladies who looked like they should be lunching at the Ritz
After trading a polite smile for a disdainful glance from the lady companions, I decided the first thing to do was get myself a cup of green tea. I discretely looked to my lady companions, and one lady passed a dark black pot to the lady next to me. I thought it must by a gentle hint and grabbed a dark pot near me for a teabag, but it was filled with green powder the colour of wasabi. Weird, I thought to myself, dehydrated wasabi to put in my soya sauce… I continued exploring and opened another pot and saw a mountain of fresh sliced ginger. I grabbed a small clean dish and piled it up with ginger.
I looked at the ladies again, but they opted to ignore me this time. I looked at the elderly man who had sat on the other side of me, but he was busy reaching for sushi already. I glanced around and spied a soya sauce bottle hidden away on the counter, and grabbed that and poured some of that into a separate dish next to my ginger. I was deeply puzzled – where was the tea?
Figuring, I had soya sauce, I put a delicate tiny spoon of the dehydrated wasabi, into my soya sauce and stirred it around. It wasn’t dissolving very well. I became vaguely aware that the ladies were glancing at me and tittering in subdued horror, but I just ignored it and reached up to grab my first plate of sushi from the sushi conveyor belt.
With fresh sushi in front of me, I picked up with my chopsticks and dipped it into the soya sauce and lifted it to my mouth to take a first bite – and that’s when I saw it
A dish of wasabi. On the sushi train.
As the sushi began to dissolve in my mouth, I was distracted momentarily by the odd flavour of my wasabi-laddened soya sauce; it was quite sweet but not at all unpleasant. Not hot though, which surprised me. And then it sunk in.
I had mixed green tea and not wasabi with my soya sauce!!!
The ladies were tittering furiously beside me, then abruptly downed all their chopsticks, discretely dabbed their lips with their paper napkins and with a furious look at me, got up to pay the bill. I turned to the gentleman next to me. I got his attention then pointed at the black pot then my mug of hot water. He nodded. I’d mixed green tea powder into my soya sauce not wasabi! No wonder the ladies were commenting amongst themselves – they must have been wondering what we foreigners do to destroy their delicate flavours overseas!