Because cheating ruin the achievements

The Honest Achievers group is a group of people who believe that achievements should be done in a fair way, during real gameplay. Being a part of this group means that you accept to not do achievements with people who help you by standing passive on an almost empty server to let you do taunt-kills etc. You also accept not to help others doing this. By accepting these rules, others can look up to you and your cool achievements a say "Wow, he/she earned those!".