Yes it is, unless those clothes are bought with company money. Did you overlook that or do you just think that it doesn't make any difference?
I didn't overlook it. Salaries are expenses, too, and most companies at least attempt to control how their employees spend their money. Whether or not unauthorized purchases, personal or otherwise, are actionable depends on the jurisdiction.
In any case, Microsoft can approve or deny spends however they choose, within the law. While high performing employees should be given leeway in their choice of tools, most organizations don't see it that way. I'm just writing academically; I haven't actually read the memo. ;-)
EDIT: I read the memo. He used "should," which means there can and will be exceptions. Good call.