My guess is that there are three reasons that companies say 6 - 8 weeks.

1) Batching (as has been mentioned). In order to get the best price, match up needs, deal with behind-the-times technical issues, doing large volume makes sense.

2) Percieved customer service. If we're told 6 - 8 weeks and it really takes a week or two, we feel pretty good, don't we?

3) Cash flow. The merchant can wait a month or so to buy the miles. That's a month or two of cash that stays in his "pocket".

Gleff's example of Amex is on the mark. "Normal" transactions seem to post in most programs quickly. It's the "specials" (Sprint, hotel bonuses) that always seem to take "forever".