Earlier quoted context omitted.
This isn't eccentric, it's poor communications though. It could basically be summarised as: "Please provide tea making facilities: tea bags, milk, sugar and hot water. If possible a couple of cans of Pepsi (not diet or other brands please) would be appreciated." Really, the person organising the speech doesn't really care how someone takes their tea or why they don't like Pepsi or what the conditions (which are total…
I would wager that you'd end up being handed a can of Coke at good proportion of the time. "Oh sorry, they don't serve Pepsi here." Richard Stallman's way makes it clear that he does not want to support Coke. He does not want you to hand him a Coke by mistake. Since I assume that is his goal, he communicated it well. His story helps you to notice, and remember, this seemingly trivial detail.
EDIT: Make it clearer by saying "Pepsi (absolutely not Coke or any other brand)" by all means but the person buying the drink wants to make sure you get what you want and then get on with their lives, they're really not interested in the why.
It feels to me that Stallman wants to use this - as with most of what he does - as something of a polemic about what he believes. That's fine but it does make the whole document less clear about the actual detail of what he wants.