Mostly because Hasbro evidently considers all their big toylines to take place in the same universe... or metaverse... or something, but there are a few other points.
Action Force definitely takes place in the same universe as GI Joe, and started off as a series of GI Joe redecos. Before Action Force, there was Action Man, the original GI Joe redeco.
Likewise, the names Action Man and Action Force are rather similar; perhaps Action Man was an early project conducted by the British Department of Defence, and when he proved successful, they expanded the project to an entire team - a force of Action Men, if you will.
Alternatively, Action Force came first, and Action Man was the top member; he was so good, in fact, that he was selected to regularly conduct surgical strikes where sending in more than two men and a dog was logistically unfeasible.
- What about Fidget? Make it a love trident... love fork? Love chicken foot...
- It's been hinted several times that she may be in a relationship with Rikki
- I second the previous notion simply for the term "love chicken foot".