Well established precedent holds that anything inside a vehicle is considered to be within the possession of the person in control of the vehicle, not the owner of the property where the vehicle happens to be located. A moment's reflection will show that any other conclusion leads to absurd results.
While business are certainly entitled to prohibit their employees -- even those with CPLs who would be, otherwise, entitled -- from bringing firearms into the areas within the control of the business, this authority does not legally extend to property outside of their legal control and responsibilty, even if it is not outside of their legal ownership, e.g., a parking lot.
Further, a ban on employees keeping their lawful firearms locked in their vehicles while at work has the de facto effect of disarming CPL holders in their commute, as well. (Although that problem could arguably be mitigated by employers providing their own storage safes, but that might open them up to the very liablity they now do not have if the safes are in the employees' vehicles.)
This bill essentially clarifies, rather than amends legal responsibility -- as, I'm sure, any employer would insist should authorities attempt to charge the business for being in possesion of any contraband (firearms, drugs, etc.) found in an employees vehicle parked in a company lot. It should be enacted.