**I heard a while back that some people spam by buying EMDs and then use their natural traffic by redirecting it to their main site. **
Where exactly did you hear this? Can you share a link?
People often say "I heard...." There is really nothing anyone can do to respond to such statements. SEO information is spammed on tens of thousands of sites. Every site owner has an opinion. We need to understand who said it? Did Matt Cutts say it? Or was it a random webmaster?
I heard it is important to optimize the meta keywords tag. Last week I had a client share two different monthly reports from two SEO companies where their meta keywords tags were being optimized each month. Does that mean I should run out and optimize all my meta keywords tags? No. The two companies were from India and their SEO knowledge is quite flawed.
**So Google started seeing it as spam technique. **
Google's Guidelines are published. They have been revised to offer additional clarity. Furthermore, Google has a public webmaster forums and publishes many articles and videos offering further clarification.
I work with Google penalties every week. I have never seen nor heard of a site penalized for having EMDs pointing to their main site just because they were EMDs. You discounted the prior example because SEOmoz is a large domain. OK. Let's use another example. If I were to go out and buy 100 domains such as "SEOtips.net", "SEOtricks.net" and 98 other similar domains and redirect them all to my company site, Vitopian.com I would not expect a penalty.
Now, if you were to alter your question in such a way as each of these domains had one page websites on them, then they become doorway pages which is a violation of Google's Guidelines.