It is clearly a bald faced lie, coming from the least trustworthy source. If they told me water was wet, I'd double-check.
It's for price localization - sell at what the market will bear, and different regions have different price points.
Sure, but I would assume that the US is a single region for the printer cartridge DRM's purposes, but you can easily find both areas of rather high and rather low humidity in the US.
I don't think the parent you're replying to is saying that the idea that things perform differently in different climates is a stupid claim, but that it's essentially impossible that's the reason why HP region-locks their cartridges, and claiming such is indeed stupid. (Or just flat out rude and disrespectful of the support person.)
There is only one US region cartridge and the US includes Alaska and Florida, or the high plains and Florida. The difference in humidity in these regions is massive. I would need different ink in the summer and winter in my upper Midwest state.
It doesn’t pass the smell test. Humidity probably does affect printing on some level, but that is irrelevant to the HP support person’s claim about why cartridges are region locked. They’re lying about why the region locks exist, this isn’t hard to understand.
Can you explain why it is a stupid claim?
High humidity is known to affect things such as ink drying time which can affect print quality, so it isn't immediately obvious to me that it would be stupid to have different ink formulations for different climates.
Edit: note that I'm not saying that HP was telling the truth. I just wanted to know how the very idea that of different inks for different climates is apparently stupid.
Yes, in most places the humidity varies a good amount. But there are regions where it tends to be high most of the time, and regions where it tends to be low most of the time. It doesn't seem inherently absurd that an ink company might have say low, high, and regular humidity ink formulations designed for areas that are persistently low, persistently high, or neither.
You might expect this to be pointless because humidity often swings a fair amount each day, but that the way humidity affects printing is by changing the moisture of the paper. A place where humidity swings from low to high and back on something like a daily cycle is likely to have much smaller swings in paper moisture content. The paper will remain closer to the some sort of weighted average of past humidity.