If you are new somewhere and have a chance to learn, you should at least in the first time show that you are really dedicated to your studies and want to achieve your goal. It's the same if you have a new job; if you don't prove that you fit in there in the first months and you even don't seem to care much about your job, you are likely fired by your boss again, even if you are highly intelligent.

It's a privilege to be educated as an AS (for free, by the way), and if a novice openly shows that she is more interested in the Warders than in learning, she simply doesn't fit in. After all, Else's behaviour also wouldn't be a good example for all the other novices, who truly dedicate themselves to study.

So, it's ridiculous to blame Siuan for that decision, especially since TG wasn't that imminent at this point.