I'm not sure if you are nit-picking on the definition of "standard", but this is not correct.
A friend of mine used to work for Marriott procurment and the brand guidelines/standards/requirements DO specify the brands that have to be supplied, not only for toiletries, but for coffee and even jams!
Marriott has very tight brand standards and enforcement, which is why you can pretty much rely on finding the same stuff at different properties, at least in the US. I did notice some regional/international differences - not sure if that's different guidelines or enforcement...
Where it's listed in the standard, yes, but that doesn't mean that
every brand choice you happen to see at a hotel is due to a standard.
Or it could be due to indirect standard, such as "only buy supplies from WhoeverCorp", and WhoeverCorp only carries brand X.
The OP mentioned Pepsi here and Coke there, and is there really a standard that says one or the other, or just a standard that says "any of the following brands of soft drinks, in the basic version" or something like that?
At any rate, there are way more types of standards than just what brand toiletries to use. Like, for starters, which toiletries are mandatory for a room, and then which are optional. LIke do they have to places in a particular pattern?
Anyway, that's the
easy stuff to make standard. How about room key types (swipe vs contact), that's certainly not standard. How about room lighting, that's certainly not always standard (it's hard to be standard on that except if it's new builds that are
all built the same way, like some Courtyards).