I'm actually hesitant to be too critical of GW there.
When they release a codex, they have a plan for which models they are going to release and a schedule for when they are going to release them. If another company jump in and releases their own version of that model prior to GW releasing theirs, then it cuts down on the potential number of sales left for GW, who are the ones who went to all the trouble of developing the rules, fluff etc for that unit.
It's a bit different if it's one of those models that they have simply taken too long to come out with (say for example, Imperial Guard penal legionnaires), or never will, but it's a sliding scale, and at the end of the day, I don't think it's fair that GW should miss out on cut of the profit from sales of models based on the intellectual property that they developed. Anyone making miniatures designed to be used with GW games should pay GW a reasonable licensing fee.
I actually think that would be good for the hobby, particularly the Specialist Games like Necromunda, Bloodbowl or Mordheim, which could really do with an expansion in their model range.