Last week the FIA International Tribunal reprimanded Mercedes and ruled that it could not participate in next month's young driver test at Silverstone as a penalty for using its 2013 car for three days of running with Pirelli at Barcelona in May.
The judges stated that Mercedes had breached the rules but determined that there had been no deliberate intent.
In a statement issued just after the verdict, Horner said he "accepted" the penalty, but was more forthright in an interview with the Reuters and Press Association news agencies on Wednesday.
"What is slightly confusing is the leniency of the penalty," he said.
"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. I should think it was met with a huge sigh of relief at [Mercedes base] Brackley."
HORNER: MERCEDES IMPROVED AFTER TEST
Horner pointed out that Mercedes' tyre issues had become less severe since the post-Spanish Grand Prix test, and said its form in this weekend's British GP would be telling.
"What you have to remember is in Barcelona, Mercedes started their cars first and second and finished with one car in the points, one car out, with the highest degradation, probably, in the field," he said.
"The very next grand prix they had one of the lowest degradations. That might purely be coincidence, and I'm sure the [Monte Carlo] circuit lends itself more to that," he said.
"But in Montreal, they didn't experience the tyre issues they previously experienced, so let's see."
Mercedes boss Ross Brawn insisted to AUTOSPORT earlier this week that missing the young driver test would hurt the team's development programme, but Horner was not convinced.
"Yes, it's probably annoying for them to miss it, but it pales into insignificance compared to the benefit you would see from running your race drivers around a race track for 1000 kilometres on a circuit that has been rubbered in two days after a grand prix weekend," said Horner.
"You don't test with young drivers over the winter for a reason. You have a limited amount of test mileage with your cars and you choose to put your race drivers in because they give you the most relevant feedback."