Google has posted a warning on their Webmaster Blog about providing free review samples, they will take action against companies that do it:
"Bloggers should use the nofollow tag on all such links because these links didn’t come about organically (i.e., the links wouldn’t exist if the company hadn’t offered to provide a free good or service in exchange for a link)."
https://webmasters.googleblog.com/2016/03/best-practices-for...
"Maybe I even skirted the rules a bit by sending him a free sample of my cool toy and some rough code samples. He took it well beyond what I could. I didn't ask for anything."
Certainly seems easy enough to argue complies with:
"send bloggers free products to review or give away in return for a mention in a blogpost"
If you don't ask for mentions in blogs, that Webmaster Blog post doesn't technically apply, right? There's gray area in choosing to send free samples to people you are pretty sure will mention/link to you in a blog post, but where you don't ask them to (or place any expectation on them to). So "skirting the rules a bit" sounds about right to me. There weren't "review units", they were "free gifts". No request or expectation of a "review".
That said, as far as ethics & free gifts, this is why politicians, journalists and businesses usually have rules about accepting free gifts, often requiring them to return the gifts or give them to charity instead.
Here's a post on Journalism Ethics relating to accepting free gifts:
https://ethics.journalists.org/topics/gifts-free-travel-and-...
They got attention as well. They may well be the center of the attention for your product now, with people with questions asking them or their community first.
The trade you entered is quite common with reviewers nowadays.
I then sent several folks a free sample of a thing, and also rough code, that costs me $100 or so.
And the gamble paid off. One of them ran off with it. It benefitted them pretty much zero as far as I know.
I totally gamed the system. It was not natural in any sense. I got ~3000 sales, and maybe $10k of upside, which was disproportionately in my favor. Most of the magic was this person's fruitful efforts to take my rough idea into something popular. But I like to selfishly think nobody really "lost" in this little microcosm.