This is not legitimate under the terms of the cc by-sa 3.0 with attribution required license used by StackExchange.
In fact it is a bait-and-switch style operation. Although one cannot tell it from the summary results returned by a Google search, per the OP's suggestion, the referenced web-site does not actually deliver the content that Google had spidered. Instead one is forwarded to an unrelated shopping site.
Even if it did deliver the spidered content, it would not be in compliance with the requirement to link to or otherwise acknowledge the original source for that content. Instead the operation of readquestion.com replaces links and references to StackExchange with self-serving links to their own URLs. So the practice is a copyright violation as well as bait-and-switch tactic.
Google is interested in identifying content abusers of both kinds, but particularly sites which show one content to search bots and another to humans who follow the links that show up in search results. Google calls this sneaky redirects and cloaking.
Since the shopping sites presumably pay for "referrals" by readquestion.com, one appropriate place to report this is:
Google: Report spam, paid links, or malware - Search Console Help
The StackExchange Powers that Be are probably in the best position to complain about licensing/copyright violation, since they can do so on behalf of the collective original authors across SE sites.
Added: I have submitted a Google Search Console spam report for a readquestion.com page that scrapes one of my answers, noting the cloaking, sneaky redirects, and unlicensed content (copyright violation) issues:
The site readquestion.com uses cloaking and sneaky redirects to hijack search results based on extensive content scraped from stackexchange.com in violation of licensing terms. I authored the answer in specific page above.
Google's central concern here is the integrity of their search results. Thus it is more important to them that a website is gaming their search algorithms than that my copyrighted post is being pirated.
Here is prompt reply I received from the Google Search Console Team:
Spam report successfully submitted
Google has received a spam report for site http://math.readquestion.com/questions/1815457/compound-quantifier. We will investigate this spam report, prioritizing by search result impact, and take appropriate action if we uncover abuse. We appreciate your taking the time to help us improve our service for your fellow users around the world. By helping us eliminate spam, you're saving millions of people time, effort and energy.
Added 2: I've also reported the matter to Stack Overflow (as current branding has it), following the links in the Meta.SE Q&A linked above.
Here is the prompt reply I received from the Stack Overflow Team tasked with pursing such reports:
Thank you for reporting this content. The information has been passed along to the team within our company which handles such issues. It's the diligence of users like you that helps us stay valuable!
Please note, bringing these sites into compliance (or getting them to no longer serve our content) is often a long and arduous process. You may not see immediate results. However, rest assured that we're working on it.
Due to the nature of this request and the amount of time it takes to process them, it is unlikely that you will receive any further follow-up emails regarding this issue.
Added 3, etc.
I search with Google from time to time for scraped content. So far sites I found which reproduced content of mine either from Math.SE or StackOverflow without proper license notice and/or attribution are:
I'll add to the list as I find more, and I will report these to StackOverflow. If sites happen to be page-ranked higher than our original (StackExchange) content, I will report that to Google as well (since they want to avoid giving a higher rank to scraper sites).