You can buy the black caps for a few bucks. Kits come with the cheap press on white caps.
Not according to the Nikon website! I'm not sure about the legality of advertising and publicly stating what's in the box and then packing something different!
Nikon Z50 Two Lens Kit link (click What’s Included):
https://www.nikonusa.com/en/nikon-products/product/mirrorless-cameras/1632/z-50.html
I get you want the black caps, but are you really talking about legal nonsense over a few dollar retail.....and probably a few cents to Nikon.......lens caps? OK lets talk about legality. Just about every manufacturer, retailer, etc, will have, somewhere, a legal policy with language like this (which was taken from Nikons site after a few second search and I'm sure there's more that cover it)......
nor is Nikon responsible for any technical, editorial and/or typographical errors.
So my suggestion, FWIW, it to reach out to Nikon and appeal to them and maybe you'll get lucky. But if you start talking about the legality of such things to them like you did here, you'll probably be buying the coveted black caps.
Look, I agree with you about their cheap white caps.
But many manufacturers do this and have been for many years. It's part of why you pay significantly less for the kit than the lens separately. Stuff like this happens when a web designer who knows nothing about their products says "OK, the kit includes this lens so I'm going to copy and paste from the product page onto the kit page". To start talking about the legality of such things is just silly.