Ah, right; let me regroup in this way.
The way Crafting (ie, obtaining a focus or enchanted item for a caster character) is built into the rules, Crafting (the subset of Thaumaturgy) is largely irrelevant. Per RAW, you gain access to FI/EI simply by having a power that provides slots: Channeling, Ritual, Evocation, Thaumaturgy, or Sponsored Magic. If you have only Channeling (Fire), for example, you still have two FI slots and can fill them as desired, despite not having Ritual (Crafting) or Thaumaturgy. And such a character could make new items at the relevant milestones, which implies that all spellcasters can Craft items, with or without the Crafting portion of Thaum. There are no RAW limits placed, so presumeably a Channeling (Fire) caster could have a focus that adds +1 to offensive spirit spells, though it's hard to see why they would do so, since it has no value for them. But they could also have one that adds +1 to crafting frequency, and that *would* benefit them, since it affects any slots they have.
That's the RAW, or at least my interpretation of them. So the non-crafter casters that you mention in your examples would be able to have crafting focus items.
That said, whether or not I'm correctly interpreting RAW, I disagree with that way of handling things; I don't think it makes sense. Instead, I think that all casters should be able to make focus/enchanted items that fit within their spellcasting abilities. So while a Channeling (Fire) caster could make a firebolt wand, or a focus that improves his fire channeling, he could not make an item that uses spirit effects. And while any caster has a capability to create magic items within their speciality, crafting focii fall within Thaumaturgy (Crafting) and should only be allowed to those with access to that aspect of magic.
Note, by the way, that the example you gave is a special case. Thematic Thaumaturgy specifically includes all functional divisions of Thaumaturgy as they relate to the Theme. So Ectomancy includes Crafting, but only with respect to the theme, and an Ectomancy could have a crafting focus because of that (though I would argue that the focus would only work for Ectomancy, even if someone else were to gain control of it).
So again, per the RAW, I think there are no explicit rules to say you can't make any item you want; so that Ectomancer you mentioned not only could have a crafting focus, but could use it to craft an (improved) wand of fire bolts. But I think that's an oversight.