There are a few reasons to this, actually:
- Drugs need not necessarily be bad. They might be medicines too.
- Drugs as such are not 'bad' or 'derogatory'.
- Most banned nicknames involve racial or gender-based discrimination.
- They also include crude language.
- It is not possible to ban every word that can be bad in one way or the other; there are far too many of those.
On the whole, "drugs" do not fit into any category, and therefore, it is not necessary to ban it. Besides, if they did, people would find another to bypass.
> "Life, uh, it seems, finds a way."