The term "milking," I believe, has always been used in a demeaning fashion towards video game series that release sequels often. I've never heard someone say "milking" in a passive or positive fashion. Even the most "passive" ones contain a hint of spite in them.
As far as why people don't like it:
1) Over saturating the market. Many consumers don't feel the need to upgrade to the newest edition if there isn't a lot of content that makes the new edition worthwhile to them. They then might be upset that the developers didn't take longer to evolve the series more, or might simply be upset because they feel a need to have the latest game in the series even if they don't truly want it. They could also just not like the series in general and hate how it sells so well with each rendition, subsequently using the term "milking" in a very despising way. Shit.. I do that with Call of Duty and I'll admit it.
2) I felt like putting a 2 here so I could have a numbered list.
