It's true that Penn and Columbia use ED to fill a substantially and unusually large portion of their class (in the mid-to-upper 40%'s). My beef with Duke and Northwestern is that they've been filling up more and more of their class during the ED round year-after-year in an attempt to artificially boost their yield (going from the 20%'s to the 40%'s, in the case of Northwestern, and with similar numbers at Duke), and it's gotten to the point where the schools are admitting close to 40% during the ED round and near 10% during RD. Columbia and Penn are NOT in this situation, which is why I don't level criticism at them as much.
Essentially, the idea is to convince applicants that getting accepted during the RD rounds is near impossible, so if you want to attend Duke/NU, you NEED to apply ED. In fact, there have been threads here mentioning that the head of admissions at Duke is explicitly telling this to applicants. It's a scheme to use fear to get inside the minds of applicants and scare them into applying ED even if they don't really have their heart set. Frankly, it's repulsingly unethical, and the sheer suggestion that UChicago's mass marketing is in the same ballpark of unethicality is ridiculous and shameful.
a middle class student who is dependent on fin aid would be hesitant to apply ED, but they would not have any qualms about applying EA. So EA pools would logically be stronger than ED pools, yet fewer are accepted. Why?
Did you not read the rest of my post? Both MIT (6000/19000) and Chicago (10000/30000) receive roughly 1/3 of their total apps from the early round. So even if they fill half their class from early admissions alone, that practice is far better than Penn, which fills up 1/2 the class from 1/8 of their total apps (4000 early apps from a total pool of 31000), or Columbia which fills up 1/2 from 1/11 of their total apps (3000 early apps from a pool of 33000).
And in fact, simply explaining the difference between EA and ED should be enough for anyone to see how EA is much better for applicants. Both financially and for the reason that 18 year olds are prone to changing their minds.
Also, I partially disagree with phuriku. If anything, Penn should be criticized more for starting this trend. My personal "beef" with these schools is that they use ED at all. (OTOH his criticism seems to stem from a personal dislike of NU/Duke.)
a couple of things you misunderstood such as my point about higher early applications at Chicago's competitors
What's wrong with ED is the implicit pressure it puts on kids to surrender their flexibility and bargaining leverage for an admissions advantage. What's right about EA is that it doesn't require kids to surrender their flexibility or bargaining leverage.
While I agree that ED at highly selective schools can be a viable option even for applicants who need financial aid, the fact is that most of those applicants don't think so. Almost every EA college, including SCEA colleges, attracts more applicants than any ED college.
...as applicants can reject the offer and toss their hat in the RD will all other schools.