Now we know who to blame for the economy: Taint! Grab him!!!
All kidding aside, that is messed up. I haven't looked at the Maxx Trim offer, nor am I an expert, but I doubt they have any grounds, other than the scare tactic.
It is not like Trainn just pulled them out of a hat and Maxx Trim had no idea they were on Trainn's site - they have a business deal. And you didn't break the T&C of the trial (I assume). Trying a trial offer and canceling it if you do not like the product/ service is NOT fraud. (Maybe if you signed up for Max Trim over and over, then yeah, but you're not.)
If I were you, I'd forward that email to Trainn; re-reading is getting me pissed off for you. The third paragraph all but accuses you of fraud, and that is far from cool.
Honestly, if *I* received that, I would reply back (not call - I like to have things in writing) and call them out, so to speak, on the fraud accusations. And I would openly CC the various consumer blog (like theconsumerist.com) and even a few local news channels (usually every major city has one that does their "helping the little guy against big business" type segment). State that, even though you went through Trainn, it was never your intent to defraud them and that you were trying out their service, following their rules, etc. and ask if they accuse all their customers of fraud.
Well...you get the idea. But I would definitely forward this to Trainn and The Consumerist.