Paulbot has explained it quite well - as you said jaydisc, one must look at the intent. Action Masters were intended to be part of the mainstream toyline and new incarnations of those characters.

Robot Heroes Optimus Prime is simply a non-transformable figurine that represents the Optimus Prime character who can transform - thus the rationale that it doesn't count as a Transformer action figure. Action Master Optimus Prime on the other hand, is the final incarnation of Optimus Prime in the Anglophone G1 continuity!

In the toy continuity, it was explained that Optimus Prime travelled through a black hole in search of a new fuel source (hence the "Nucleon Quest Convoy" toy) and returned with the discovery of Nucleon which replaced Energon as the new fuel source for Transformers. But they eventually discoverd that Nucleon made them lose the ability to transform, so they made new weapons/vehicles for themselves that could transform to compensate.

Nucleon Quest Convoy

Apparently going through a black hole made PM Prime turn black.

In the Marvel Comics, many Transformers had been slain by Starscream during the Underbase saga, and although Grimlock, Jazz and Bumblebee (and Starscream) had been revived as Classic Pretenders by Ratchet, Grimlock desperately wanted his fellow Dinobots to be restored as well. With Ratchet presumed KIA he had to find another way to bring them back to life. He found out about some miracle energy source on a planet called Hydrus Four. He sought permission from PM Optimus Prime to go there and retrieve this energy, but Prime denied his request. So naturally Grimlock defied Prime's orders and took the Ark to Hydrus Four where he tested the Nucleon on himself first - discovering that it instantly made him feel more powerful and more alive - and then used it to revive his Dinobots. The Dinobots then pumped every stasis pod aboard the Ark with Nucleon. Wheee. The Nucleon eventually started locking up some of Grimlock's joints - giving him symptoms similar to arthritis - and he eventually went into a chrysalis stage. After bursting from this chrysalis, Grimlock had evolved into a full Action Master where he was immensely more powerful in robot mode, but had utterly lost the ability to transform into Dinobot mode. Grimlock began to fear that when the other Dinobots would eventually go through the same stage, whether they would thank or curse him for using Nucleon to restore them... but the G1 series was axed before this question could be answered. Other Action Master characters we saw in G1 included Krok (Bludgeon's 2nd in command) and Optimus Prime, who was not made into an Action Master by Nucleon, but by the Last Autobot who created AM Prime with Hi-Q (whose mind had merged with that of the late Powermaster Optimus Prime).

Action Master Grimlock beats the crap out of Fangry


Action Master Optimus Prime


Action Master Krok was Bludgeon's 2nd in command of the Decepticons


Action Master Banzaitron - a lethal assassin ("Cybertron Chronicles")