not legal advice -->
Believe it or not, the US actually has some of the best laws when it comes to defamation... maybe not when it comes to trademark/copyright, but when it comes to defamation cases, the US rocks.
First off. In the US, the onus in a defamation case is on the planitiff (to prove a statement false), while in most countries, the onus is on the defendant to prove a statement true. This is because of the 5th amendment. Some statements cannot be proven false. These are called "opinions". Where they can be proven true, they are called "fact".
Secondly, in the US, courts differentiate between public and private figures. When a plaintiff in a defamation case is a public figure, or an issue concerns a matter of public interest, a plaintiff not only has to prove a statement false, but also that the defendant made the false statement with full knowledge of it's falsity (or at least having doubts as to the truth of the statement). This is known as "actual malice".
Of course, regardless of where a website is hosted, if a program sues you, they're going to try and take you to court locally, not where you live, so it really doesn't matter what country the server is located in. If you get threatened by a program, my non-legal advice is to speak to an attorney on the matter (preferably one in the state the program is in... Utah in this case).
<-- not legal advice... neither is this -->
Case law:
"Abuse" found to be opinion (and protected as such): People for Ethical Treatment of Animals v. Bobby Berosini, Ltd. 111 Nev. 615, 624-625 (statement that animal trainer regularly abused his orangutans found to be opinion).
"inflicted psychological damage" found to be opinion: Campanelli v. Regents of University of California, 44 Cal.App 4th 572, 579 (1996)
Statement that a county officer "abused" his office was opinion: Slawik v. News-Journal Co., 428 A.2d 15 (Del. 1981).
... and lots more where that came from... As I read the letter they sent you, the only specific statement they imply to be defamatory is "Abuse", which is opinion (see above cases). The blanket demand for you to take down the whole site without pointing to a single statement they allege to be defamatory is absurd, and soely intended to silence you as a critic. My non-legal advice is to write them back asking for a clarification as to which statements, specifically, they allege to be defamatory, and ask them to provide "proof of falsity". Offer to provide an apology if they can do so.
<-- not legal advice
