One reason I could think of is that real humans only read a certain number of tweets per day. Showing ads to users that "read" large numbers is showing ads to robots which, sadly, do not convert very well.
Current rate limit is about 20 minutes of use for me. Remember that it loads a couple dozen replies at a time if you click on a tweet, so 600 tweets is really not much.
Yeah, this basically restricts users to viewing a handful of popular threads per day.
600 posts is about 10 minutes of browsing for me. It seems to be counting replies in large threads to the limit. So even if you make a post with a premium account your content won't be read by the vast majority of your followers.
It's like how scams put in typos on purpose to save effort. If I was an advertiser I'd really want to target people who donate $8 a month to Twitter.