Cop out answer: The same reason anyone else would, i.e. that's not a particular reason to exclude them as against anyone else.

Best answer I can come up with: they are laboratories for experimenting with known space sentient species. After all these are species that evolved from programmed good yeast, so the outsiders may want to figure out how that works and if there are any nasty surprises hidden in their genomes.

Which is an interesting idea I think and I wouldn't have though of it if you hadn't asked, so thanks!