Some television channels follow laws, or at least rules, that commercials played during a children's series cannot directly promote toys based on the series itself. The viewer is more likely to see such commercials played in a similar time slot during some other show. This does not always relate to other kinds of promotions, such as contests.
This probably results from media watchdog-pressure during the early nineties, when parents became sick of the blatant Merchandise-Driven aspects of shows their kids were watching (which parents pejoratively referred to as “half hour commercials”). Many shows during this period, like Transformers and He-Man and the Masters of the Universe (1983), were actually conceived alongside the toy line solely as tools to promote toy sales, as opposed to more innocent times when toys were created to boost show ratings. It certainly does not affect programs for older audiences, which at least manage to hawk their newly released DVD volumes.
Pragmatically, this is slightly bad business; Japanese television certainly has no such rule in regards to anime, which peddles tie-in merchandise shamelessly.
In the US, this law is enforced by the FCC and extends to showing real products in the show itself, which, interestingly, in Yu-Gi-Oh! and Pokémon, led to (respectively) all the text being wiped off the cards in the American version so they don't look like the real ones, and prominent appearances of Pokémon plush toys and even Poké Ball icons being scrubbed so as not to resemble plugs for actual merchandise. While it's a big advertising loss for the shows, it is less obnoxious for the parents, at least.
An example of a notable aversion to this trope: In 1997, Fox Kids aired an ad for a VHS release of Magic Adventures of Mumfie during the final airing of the show on the block. Although most stations weren't fined by the FCC for this, one Fox station actually was. Around this same time, syndicated children's programming blocks would frequently run an ad for a singing Bananas in Pyjamas toy. When one station chose to run it during an episode of Bananas itself, the FCC filed a complaint.
On the other side of the pond, early in TV's life, the UK broadcast regulator forbade adverts which featured actors also seen in the programme the advert broadcasted during. This was later dropped following the rise of multi-channel television and channels purely showing syndicated content.