It's based on the idea that the only reason people do fan subbing here in the states is because the shows we like in the tokusatsu realm are not readily available here in the states. Purchasing licensed products here in the states showcases that there is clearly a market for it here. Shout Factory have been pretty successful with Zyuranger, Dairanger and now are going to release Kakuranger soon.
Granted it does sorta take away from all the hard work that was done in the past, but then again I rather like that this stuff is being made available at all. I still have the box sets of Iron King and Super Robot Red Baron that was released by BCI Eclipse almost ten years ago, and I have the Ultraseven release that SHout Factory did also, and also have Kikaider from Generation Kikaider.
I am also planning on purchasing the Shout Factory Sentai releases for a friend of mine who is a fellow Ranger fan, only because I already have the fan subs of Zyuranger and Kakuranger, and it's not very likely that the official subbed version of Kakuranger is going to be as good or creative with the fan subs the way GUIS were when they worked on Kakuranger. Besides, it'll mostly likely be at least twenty years before they finally get to Gokaiger if things continue the way they've been going and they release one official Sentai program that was adapted into Power Rangers a year. But it also means that we won't be getting official releases for Go Buster or ToQger either, since neither of those shows were adapted into Power Rangers.
Anyway, as to whether fan sub groups should focus on pre Zyuranger programs or not, that's really up to the groups. With that said, I am happy that amongst the pre-Zyu shows released so far have come largely from GUIS, who have done, Maskman, Bioman, Liveman, Jetman, and are currently working on CHangeman and Flashman. Rampage Fansubs are working on Goranger in collaboration with LovexCare, who I think are also working on Turboranger. GaoSoul Forever worked on Sun Vulcan and finished it before they went kaput. I don't think anyone is working on JAKQ, or Denziman or Goggle V, and Battle Fever J is available, but mostly as a scrub of the subtitles used on Hawaiian TV, so it's not clear how accurate the subs are for that show are.
It's all a matter of personal choice, but it would be nice see some of these shows fan subbed in the near future.