You are right, it is not legal.

It's copyright infringement if you don't have permission.

Probably, most people don't have permission, so they are infringing on copyright.

But, some manufacturers will allow you to use images straight from their websites, catalogs, etc., if you are a reseller. Some may mention this in their terms of use; others will only tell you that if you are signed up as, or purchase from them as, a reseller.

Some may allow you to use their images in reviews and in other related content. Some even provide images to be used in such a manner.

But, that all varies by company, so if you plan on using any images, you would need to check with each company as per their guidelines and usage restrictions. Unless you get permission, which includes permission granted through their terms of use or other usage statements on their sites, don't use such images.

The bottom line is that, while many people are not authorized to be using the photos they are, that's not necessarily true in all cases.