yeah, you wanted it so here you go, you're the one who didnt pay attention, suck it now -
1. Alpert told that Ben wouldnt remember, but based on what ? based on the healing pool - the same pool that had healing properties but NOT the memory wipe ? (lots of people healed, and still aware of everything)
2. Ben was fully aware of his previous life with Dharma being himself that told Alpert he didnt want to go back to Dharma, he wanted to be the Other
3. Ben's actions do not imply that he actually remembered being shot by Sayid, and it seems so as if he had forgotten.
so, what you want to say actually is another of those "fancy" explanations people provide today:
Healing pool heals people. but, it can also randomly wipe someones short term memory, with no one else than Richard Alpert knowing when the randomness will occur.
on the other hand, we DONT KNOW how richard can do that, knowing that he only received immortality from Jacob, so that also is left for speculation.
or, you're going to say something like - Jacob did it.
let's explain everything with - Jacob did it, shall we ?