Is this style of marketing legal in US? In the country I am in it is quite likely to be illegal due to being deceptive. In my country being technical correct doesn't help if misleading.
(If these companies signed up for the service then or course it would be legal to claim that.)
Logos are fine as long as you have an agreement in place, but you run afoul of a number of laws by using another companies logo without their approval. Although, it seems this happens all of the time with various companies.
As for myself I turn down every request for a logo, I like and use your product but I don't really care if you cut me a small break for using it.
I'm not sure whether this type of marketing being legal in the US is as relevant as is this type of marketing legal in Israel since it is an Israeli company.
Maybe it differs from company to company and country to country, but at least the company I work for would hesitate enormously before purchasing a service from a company which immediate appears to do something that is illegal in many countries.
This company (is there a company behind it, I couldn't find those details) markets to countries outside of Israel I assume.
Any reasonable person would read "Companies we monitor" as "Companies that pay for and rely on our services", if that's not your relationship with the companies, then it's just using deception, and probably also violating trademark
I have now changed this to include only logos of companies with whom I am directly involved, who are actively using my service to monitor their SSL certificates, and who have agreed to have their logos appear on my landing page.
Of course these orgs have their own infra monitoring, but technically anybody can monitor them and then claim to do so.