Unfortunately that is not always  the case.

In the early days of USB drives I acquired quite a few from a combination of sources, including ebay and Aliexpress and promotionally branded freebies.

In the process I found myself with more than a few that would store less than the capacity that they reported to the host hardware. They would gladly accept the writes, but when you went back to read the data it just wasn't there (beyond a capacity many times less than what was on the label or reported by `df`).

As the field has matured and USB drives replaced floppy disks and then writable CDs and then writable DVDs for tasks such as file transfer and Linux installation, the situation has stabilized and prices have dropped enough such that there's no real value in buying something not from a recognizable brand. Apparently the scam has spread to Amazon and bogus drives are still being sold. 

Generally speaking, drives bought in most brick-and-mortar stores should be OK, as well as Amazon or Walmart online purchases of mainstream brands such as Sandisk, Lexar and Kingston. I have all three and they work fine. I would treat anything received as a promotional item to be suspect until I had verified capacity. And I would stay far far away from eBay or any Asian online source.

When in doubt, a suite of tools for Linux known as F3 (Fight Flash Fraud) is explicitly purposed to detect fakes. And, as always, something that looks too good to be true usually is.