I always thought it was pretty clear that Ashley was the obvious "low hanging fruit" "beat you over the head with this" introduction to the race relations storylines. Conversely, the Tali / Geth one is a bit more contemplative, smartly wrapped in a sapient "What is life" package and then wrapping that in "Hey, even though the Geth are a race of sapient beings, they're being mind controlled". I think the conclusion most people draw is that there is widespread understanding of Tali's prejudice, but it is what it is, good reason or not.
Would the Krogan have conquered the galaxy? Did the Salarians have to do what they did (and then did again under Mordin)?
Ashley isn't irredeemable, quite the opposite (this should be obvious, given how early on she's introduced). Ashley is a test to see which parts of the race relations flowchart you are willing to engage with. The game *forces* you to confront at least some aspects of race / species relations, whether its through your engagement with it, or your choice to ignore it.