I'd be curious to get examples of actual successful lawsuits resulting from A/B testing! Regarding the legality of it, in the US it is illegal to perform price discrimination when the intent is to directly harm your competition ([1], [2], [3]). However, first, Robinson-Patman does not apply to services [4], and secondly this is trying to avoid predatory pricing, and the supreme court ruled that price differentials are prohibited when the price differential "may be substantially to lessen competition". Similarly in the EU, Article 102 c) of the Treaty on the Functioning of the European Union prohibits price discrimination for companies in dominant position. It has however been significantly relaxed in United Brands v. Commission [5], where the court recognized that a dominant firm may charge different prices to reflect the competitive market.

In short, price A/B testing is legal and common practice for most companies, and VWO's article seems a bit too quick to jump to declaring it illegal.

[1] https://en.wikipedia.org/wiki/Sherman_Antitrust_Act_of_1890

[2] https://www.law.cornell.edu/wex/Clayton_Antitrust_Act

[3] https://en.wikipedia.org/wiki/Robinson%E2%80%93Patman_Act

[4] https://www.ftc.gov/tips-advice/competition-guidance/guide-a...

[5] https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A...