If increased intracranial pressure(icp) is contraindicated in lumbar puncture, why one of the indication of this procedure is to decrease the icp?

Depends: Lp is contraindicated when the icp is due to some process that is creating a midline shift in the brain and sudden loss of pressure can cause herniation of the midbrain and consequent death. You can reduce some icp with LP but better to place intraventricular drain and then decide whether permanent shunt may be necessary.

Different situations: It depends on the clinical situation. If there is a mass lesion in the brain that causes a differential pressure between the supratentorial and infratentorial areas, lumbar puncture can accentuate the difference and lead to herniation of brain downward. If there isn't this differential (communicating hydrocephalus), a spinal tap can decrease the pressure uniformly throught the CSF system.