The TrustPilot makes money by offering merchants to integrate reviews with their e-commerce system. This results in slanted reviews for these merchants compared to those that do not pay. Since most people post reviews only if they are dissatisfied, a website that does not pay TrustPilot will usually have worse ratings than a website that does, even with the same level of customer satisfaction.
The fair thing to do would be to take the effect into the account when computing the "overall" rating, boosting the ratings of those websites that do not pay TrustPilot. The way the system is, the TrustPilot's business model is a modern-day blackmail.
N.B. I am not a business owner. I just wrote a few reviews on the website, and then decided to check "For companies" link.
You are right that dissatisfied customers might be more inclined to write a review by themselves than happy customers are. However, these companies will usually have less reviews.
The way the Trustscore is calculated accounts for representativity. If a company has received five negative reviews, the score will not deviate much from the middle score because five reviews is not sufficiently representative.
If you like, you can read more about the Trustscore at this link http://www.trustpilot.com/About/FAQ