The Silver Arrows were reprimanded and banned from competing in next month's Formula 1 young driver test after secret 1,000km trials in conjunction with Pirelli came to light.
However, Horner does not believe that Mercedes's punishment was severe enough to deter other teams from committing similar offences.
"The problem with the penalty such as the one Mercedes have been given is that it is not a particularly strong deterrent to break the sporting regulations," he told Reuters. "I should think it was met with a huge sigh of relief at Brackley (Mercedes HQ)."
Mercedes escaped suspension from the upcoming British Grand Prix and a possible points deduction in the constructors' championship following a ruling by the FIA's international tribunal.