Mildly Infuriating

It's not fabricated, the machine dispenses a fixed quantity, expecting ice.

Which is customized per drink based on specifications arbitrarily selected by the franchise. It's literally a fabricated value. If you think they don't adjust that and don't think it's configurable I've got a bridge to sell you.

Further: it's a coffee shop. Custom orders are not foreign. Easy ice isn't even that unheard of. There were a variety of ways to approach that situation as the cashier and the 'nope' that OP got was not one of them. Hell they could have offered to cut it with regular coffee to top it off and not used the machine. Kid behind the counter was being lazy and made an excuse. Simple as that.

A subway sandwich is made to have a fixed quantity of deli meat, even if you ask for no other toppings.

This is apples and oranges. Meat is a high cost item with low margin. Drinks are literally the opposite of this. But to your point if someone asked for double meat you'd just tell them there's a charge for it... because it's quite literally built into the POS system. Extra shot of espresso? Yep that's there. 2oz more coffee? Not there. Why? Because it costs more to pay someone the extra 2 minutes pushing the button in the POS system than it does to just pour the drink.

In the case of the coffee, they'd have to mix a second portion of drink in s, m or l, and hope it fits in the m cup.

From what I read it doesn't sound like pushing the button a second time would have overflowed the existing cup and honestly? There's a spill tray. This is low effort and the clerk cba to make even the most basic effort.

Indeed folks should prefer small coffee chains. The product is way better and the servings aren't portioned by a machine, and asking for adjustments at such a place doesn't put the employee in a spot of potentially getting in trouble.

I promise you the manager wouldn't have batted an eye at filling the cup, and would likely be more pissed if a complaint came in over this exact incident. I can almost hear them saying "WTF were you thinking"... and "if you weren't sure come get us."

You don't know the Starbucks employee's life, and them following the rules isn't choicefully being a "drone". They just want their wage, and want to serve customers shit off the menu.

Find me a Starbucks (or any coffee chain) that doesn't take custom orders on... their entire menu... The drone comment was quite frankly accurate. If you can't do basic problem solving why the fuck are you there. The customer could just push the button and... to that point this is why a LOT of drink machines are self serve.

Arguably they have more "common sense" to keep their manager off their ass by just serving from the menu, rather than doing custom stuff

I covered this above. There was more to gain and less to lose over the mere pennies that second button push would have cost... and checking with a manager is quite literally part of the job.

edit:

speaking of fabricated - I checked the prices (see other post)

The iced large late actually costs more than the hot large late lol. The prices are literally made up in this case. Arguably the only difference in cost is... the ice... which he wanted less of so....