Money. It makes money for the government through taxes. That is
the only reason it is legal...because of the huge amounts of money
it brings in. And that is exactly why marijuana is
illegal...because they would have no way of controlling the
producers (tax payers).
However, to be fair nicotine (consituent of tobacco) does have accepted medical potential. For example, nicotine patches are used to potentiate the effects of haloperidol and pimozide in patients suffering from tourette's syndrome. So it would be more fitting to label tobacco as schedule II (accepted medical use and highly addictive), the same class that includes methylphenidate (Ritalin), cocaine, methamphetamine (Desoxyn), morphine, etc. However, this is not the case.
It is interesting to note that most Schedule I substances do have medical uses (whether generally accepted or not) but are considered too "addictive" or "dangerous" to give to the general public. For example, heroin has ample medical uses since it is broken down into morphine (schedule II) to be used by the body, but when injected this substance can be extremely habit-forming. Other schedule I examples include LSD or psilocybin (mushrooms) which can be used to treat cluster headaches (aka suicide headaches), but are relatively non habit-forming. In summary, the scheduling of substances does not reflect so much on how addictive a substance is or what medical potential exists. Instead, substance classifications are more of a reflection of social attitudes and taboos surrounding them.