為何在香港吸毒不犯法,而賣毒品就犯法?

There are two sections of laws of Hong Kong relevant to your question.

s.8, Cap.134 Dangerous Drugs Ordinance.
1) Save under and in accordance with this Ordinance or a licence granted by the Director thereunder, no person shall-
(a) have in his possession; or
(b) smoke, inhale, ingest or inject,
a dangerous drug.
(2) Any person who contravenes any of the provisions of subsection (1) shall be guilty of an offence...

s.4, Cap.134 Dangerous Drugs Ordinance.
1) Save under and in accordance with this Ordinance or a licence granted by the Director hereunder, no person shall, on his own behalf or on behalf of any other person, whether or not such other person is in Hong Kong-
(a) traffic in a dangerous drug;
(b) offer to traffic in a dangerous drug or in a substance he believes to be a dangerous drug; or
(c) do or offer to do an act preparatory to or for the purpose of trafficking in a dangerous drug or in a substance he believes to be a dangerous drug. (Amended 37 of 1980 s. 2)

(3) Any person who contravenes any of the provisions of subsection (1) shall be guilty of an offence...

The idea of control of dangerous drugs in Hong Kong is by way of licences to those who has good reasons to use the drugs e.g. pharmaceutical companies, hospitals and doctors. Drug trafficing is an offence for obvious reasons. Some people take drugs for good reasons, e.g. patients for medical reasons. For others who smoke and inhale drugs for pleasure, they do not have a license and contravenes s.8(2) of Dangerous Drugs Ordinance and it is also an offence.

But it is easy to understand that the prosecution cannot charge a person for contravention of s.8(2) unless the defendant is caught in the middle of smoking drugs. Most of them are caught when they are found to possess a small amount of drugs. So they are charged with s.8(1) for possession of dangerous drugs without licence.

吸毒和賣毒品都犯法.

吸毒是犯法的啊