Search engines set up guidelines that help them extract billions of dollars of ad revenue from the work of publishers and the attention of searchers. Within that highly profitable framework search engines consider certain marketing techniques deceptive in nature, and label them as black hat SEO. Those which are considered within their guidelines are called white hat SEO techniques. The search guidelines are not a static set of rules, and things that may be considered legitimate one day may be considered deceptive the next.
Search engines are not without flaws in their business models, but there is nothing immoral or illegal about testing search algorithms to understand how search engines work.
People who have extensively tested search algorithms are probably more competent and more knowledgeable search marketers than those who give themselves the arbitrary label of white hat SEOs while calling others black hat SEOs.
When making large investments in processes that are not entirely clear trust is important. Rather than looking for reasons to not work with an SEO it is best to look for signs of trust in a person you would like to work with.