Editable tweets is one of these things that everyone gets behind until it gets used for the wrong reasons.
If you tweet something that many people disagree with, you are currently given the options of accepting the disagreement or deleting the tweet.
If you tweet something that many people disagree with, but are given the option to edit the tweet, you can then edit the tweet to reflect badly on the people that interacted with your initial tweet.
Say you tweeted "Like this if you love cats". You garner 15k likes. You then edit that tweet to say "I am attracted to children". Every one of those 15k people that liked that tweet have that on their profile for everyone to see.
If the edit function is ever enabled, the only fair way for it to work is if the initial tweet is edited, all existing likes or comments are automatically deleted and you start from fresh. it ould be completely unfair for people to comment on or like a tweet for it to be edited at a later date.