Maybe an analogy to sales would make this more understandable: If I was selling computers and the laptop I sold you had some serious issues, you cannot immediately turn around and call me a scammer. Because it might not be true, actually there is a more likely explanation that I didn't know about the issues with the laptop.
Now if you would talk to me instead and I would insist that there couldn't possibly be an issue with the laptop and charge you for the faulty product, you _might_ call me a scammer. (Maybe one single instance of this wouldn't even be enough. Maybe you would even need to prove that I did in fact know about the problem with the laptop)
On the other hand you can definitely leave a review saying that you received a bad product. You would not be required to talk to the company to be able to do that.
Edit: To further elaborate a bit because I think people in this thread are having some weird reactions to this ruling: It's pretty normal that you cannot go around and tell lies that hurt somebody's business. If you think about it this is very normal. I cannot stand in front of a bakery with a sign that says "The bread in this bakery contains dangerous amounts of lead" if that is not in fact true. And if nobody buys bread anymore because of my sign, I will be liable for the damages incurred.
This is less clear if my sign says: "In my opinion the bread in this bakery tastes funny." Because this is an opinion rather than a fact.
But in the case at hand we are talking about somebody calling a company a "scam solicitor". That's mostly a factual statement, you could prove that the company is either doing regular business or that they are scamming people. If the defendant is not able to prove that they are in fact a scam business, it's very reasonable to assume that he is liable. The concrete damages incurred are however debatable in my opinion.