Legalize it, tax it, and the marijuana drug wars dry up, the jails are emptied from non-violent criminals caught up in the drug war. However, the law enforcement groups hate this because it takes away a big chunk of their illegal money through seizures of cash and property on the excuse that it MIGHT be used for drug purchases.
If that is the desire, then the laws need to change first.
I would caution that certain jobs come with significant liability when performed under the influence (including both alcohol and drugs). The lack of a test showing influence at the time (as opposed to the 20+ day residuals) for pot means that pot smokers would still need to be barred from such employment, for protection of the employer.
Legal does not necessarily mean safe.