Copying an excerpt and linking back is fine. Even in PC h/w reviews we have the same system. One excerpt, 1 featured image (depending on the site) and backlink. As it is people will have to click on the link to read the content anyways. Its not a big deal. I do the same for review website that does for me. links are no follow and doesn't provide pingbacks. So the source of traffic through the site is always direct.
If you dont like it, you can ask them to remove. You're just cutting off a potential option that may get you traffic. Its like when I got 14,000 pageviews in a day few days ago. 32% were referral links- websites, and discussion forums. Its no different. Some give small traffic. some give none. Some helps to gain traction for your website that generates good loyal readerbased out of websites that have loyal readerbase. If the entire content was copied and then linked in the bottom, then that's when the problem is. That's not called plagiarism, but that's also not called fair use.