I don't understand why people yell at those that edit the valve maps, especially when the edit involves changing a well known map to provide alternate game styles.
Because it is considered lazy and unimaginative.
TF2, despite now having been updated with several maps, community maps and new map modes, is still better off with unique and fresh interesting maps. Not rehashed versions of the same official maps. The idea is to provide more variety to the game, and editing a game mode in an already released map can only forfill that need to a limited degree. Playing any particular map all the time, despite the game mode, will get boring, and nullify the maps enjoyability.
On a side note, users here spend months building, testing and perfecting their own original creations, so the mere mention of releasing someone elses work with a few flicks of the wrist to edit the mode, is an insult to all their hard work.
Generally you find those who edit an official map are the ones who like the idea of mapping but don't really feel like researching into how it all works (and subsequently never release any "real" map of their own). So the dedicated members have no motivation to make time for these people. The act of intruding upon these idea's is only to try and motivate them to do otherwise, as they are doing; producing original content that will refresh the gamers experience of TF2 to the maximum amount. This community is considered a place to learn and teach by the more active and dedicated members so the very notion of merely rereleasing other peoples work does not pass well, and is believed it should be kept to places such as FPSB, where publicity and file sharing is the name of the game. Rather than education and quality control.