Aggregators don't replace the original source they point you to it.
Engadget (and similar sites) are just parasites that extract every ounce of value from someone else's work, stuff it full of SEO spam and hope they do it fast enough to usurp the traffic too.
I do agree that they shouldn't be submitted though!
If there is any wrong doing, it is the people who post links to one aggregator (like engadget) on another (like HN).
I guess in the HN case it is likely because of the url dupe filter.