In some countries, it's called ambush marketing. In others, it's referred to as parasite marketing.
Event organizers deplore the maneuver on principle, of course, but there's a practical reason, too.
Official sponsors pay some very fancy prices to identify themselves with the event in the public consciousness. They do not like to see the impact of their sponsorship diluted by competitors who have not paid a penny extra for the same association. Sponsors expect the organizing body to protect their interests. But it's not strictly illegal to infer a connection with an event, so organizers must take a less direct route than the one through criminal court. Protection Act of 1996.