There seems to be no valid reason for the grace period of full one year
The avowed position of the central government is that it is committed to the implementation of GST in India, effective April 1, 2016. While there could be some debate and contingencies around the date of introduction, the next relevant question is if the introduction across the country will be simultaneous or could states have the ability to introduce it in a staggered manner. A look at the Constitution Amendment Bill for introduction of GST perhaps provides the answer, albeit an indirect one.
The Amendment Bill’s clause 20 provides for ‘Transitional Provisions’. This clause, worded in what lawyers describe as a ‘non obstante clause’, overrides all other clauses of the Amendment Bill to permit an existing state law for a period of one year even after the effective date of the amendment of the Constitution. The existing state law, it is provided, shall govern even if it is inconsistent with the constitutional provisions relating to the GST design.
The underlying intent seems to be to provide a breathing space to the states to transition to GST. The states are expected to get their act together and get in line with the GST design at the earliest and in any case no later than one year from the commencement of the new regime. It is in this wake that clause 20 provides that an inconsistent existing law of the state shall be permitted to operate till such period as modified to bring in line with the GST regime.
The appreciable intent of the clause notwithstanding, this provision carries the potential to legally permit inconsistency in the implementation for the GST regime for a full period of one year.