I worked as a patent examiner twice in the past. I've written on Hacker News before about how patents are granted in the US:

https://news.ycombinator.com/item?id=33506062

https://news.ycombinator.com/item?id=31197809

https://news.ycombinator.com/item?id=31883765

Why was any particular patent granted?

First, keep in mind that the claims are the legally enforceable part of a patent. The drawings, title, abstract, and specification aren't as important. The title and abstract could be really vague and sound like they cover far more than the patent actually does as described in the claims. That's often intentionally done by attorneys because they want as broad coverage as possible. Usually only the claims are amended to be more narrow, so the broad sounding abstract/etc. stays.

Second, you need to check that it was indeed granted. Many times when someone's complaining about a dumb patent, it turns out that they are referring to a patent application which the patent office rejected. Give the USPTO credit where it's due. They did their job properly there.

Third, you can actually read the examiner's logic about why they granted a particular patent in many countries. In the US, you can look up the application on Patent Center and click on "Documents & Transaction" on the left. For the patent you mentioned, that'll take you here:

https://patentcenter.uspto.gov/applications/17873839/ifw/doc...

Now look for the "Notice of Allowance and Fees Due (PTOL-85)". This document will describe why the application was allowed. Usually it'll highlight the difference between the patent claim and the prior art. In this case the "Reasons for Allowance" section of the document seems pretty clear about why the examiner allowed this application.

Fourth, even if the examiner was in error, don't be too harsh on them. Being a patent examiner is a really hard job. They have a quota system and the quota is simply unreasonable. It's impossible in my opinion to do consistently high quality work for most patent examiners with the current quotas. If they can't find prior art in the allowed time, right or wrong, they'll probably have to allow it even if they don't feel that's right.