Unfortunately the majority of these websites are legal, on the grounds that they are (officially at least) not claiming to be the official eTA website, but instead a website that will help you obtain your eTA.
At some level, this is no different to companies like H&R Block that assist you with filing your tax return - you can do it yourself for cheaper, but the companies are offering a service to do it for you, and obviously charging an additional fee to do so.
In general these websites will include text that states they are not the 'official' website, but this is normally in small print towards the bottom of the page. For example, the one you've linked to above states :
eta-canada-visa.com is intended to accompany the natural and legal
persons in their paperwork travel. eta-canada-visa.com is not
affiliated to the Canadian government authorities or to the website of
the Canadian government. The price charged is 44.50 Euros and covers
the costs to the immigration service which is $7. You can get your
request on the official website of the government, but in this case
you will not receive our help and assistance. The eta-canada-visa.com
site is a private website and is not affiliated to the Canadian
government authorities or the website of the Canadian government.
Which would be a fairly clear statement if it wasn't buried at the bottom of the homepage where nobody will look.
As a rule, these websites DO submit your application, and you DO receive an eTA, so they are technically delivering on the service they are offering. Some people will recommend doing a credit card chargeback if you discovered you've fallen for one of these "scams", but in general that will NOT be successful as the website has officially done nothing wrong, and they have provided the service you contracted them to provide, at the cost they quoted in advance.
Over the years there have been examples of websites which crossed the line (eg, claiming to be the official government website, or not actually submitting the eTA application on your behalf) but these do normally get shutdown fairly quickly. If you find one of these then reporting it to the relevant Government. In the case of the Canadian government they have a number of contacts on their website, depending on the exact situation.