I dont think it's illegal, but here's something that was posted this morning.
https://searchenginewatch.com/sew/news/2241393/google-warns-seo-businesses-to-avoid-fake-reviews
In case you didn't read, google is getting better and better at finding the fake reviews with their algo updates, and they will remove them.
I wouldn't advertise this on your site. Maybe an email list, because they state in the article, they're targeting (and probably red flagging) businesses who are providing fake reviews.
My advice, would be to avoid the fake reviews. It's only a matter of time, before google will red flag anyone who reviews with a MAC address from someone who isn't local. This may already be taking place.
Unless you're talking about reviewing online businesses, and then even fakes are capable of being real.
The big thing, is not to give over the top reviews, but instead, to write with "reactions to provided services," and possibly a story.
So, let's say the service is selling Facebook Page Designs. you can write about the ordering process. And try to make it a CRITICAL review, by include something negative, that has been resolved. Even if it didn't happen. It will show that you're not just a fluffer.
"At first I was skeptical about Bob's Facebook Page Designs. But after we talked back and forth, I was confident he understood where I wanted my page to go. After three days, he emailed me the design for approval. I didn't like the font of the text, so I asked him if he could do something a little more clear with the lower case f, because it looked like a t. Only 30 minutes went by, and he emailed me three new updated designs, with different fonts. I loved the second one. He let me keep all three, and suggested I changed it up a little bit to keep it fresh. I would recommend him to everyone, because he was reasonable in price, he was fast to respond, and I loved the design!"
