Here's the thing - I'm not comfortable giving a "testimonial" for a product you gave me for free. I'm a copywriter - if you want me to write marketing material for you, it's easy. Pay me.
Outside of that, if this is something that happens a lot, how many of the testimonials in there are just garbage borne out of reciprocity?
I don't see anywhere this is against the rules, but I'd say ethically, if you've been asked to do this and choose to comply, SAY SO in your "testimonial" fakey review sales post.
I like when I can tell the difference between actual user feedback and bribed puffery. Anyone have a different opinion about this practice? Even if it's not against the rules, I'm curious what others think.