...and Uber is just a software company, and Mastercard/Visa just facilitate payments.
The specific classification of a company is irrelevant. If they've induced their own gravitational field, then they're responsible for the consequences.
Assessors are obligated to be qualified because they are obligated to their clients who pay them money in exchange for their services.
How much did the seller in question pay zillow and what was promised in return?
The fact that person a's action foo can negatively effect you doesn't imply that the law or ethics precludes a from doing foo. A must be either violating your rights, the law, or have a duty to you that would preclude foo.
Its not clear how you could have a right to silence others speech in order to earn more money, its not clear what duty zillow has to sellers, and its not clear that the law actually precludes zillow from underestimating the value of your property.
In fact it looks an awful lot like none of those apply, and further that allowing anyone who feels like their property is undervalued to use threat of a lawsuit to silence discussion would preclude anyone from providing public analysis of property values whether accurate or not and thus harm the public at large. Real harm unlike the unproven harm previously discussed.
The specific classification of a company is irrelevant. If they've induced their own gravitational field, then they're responsible for the consequences.