Big companies donate said money ( primarily ) to avoid taxes. Big companies are smart - they know exactly how to spend their profits - on equipment, on building upgrades, on capital expenses, on donating to charities, etc.  ( secondarily ) these big companies get some good press by saying 'look, we donated money to x y and z'.

If oregon penalizes these big companies by adding a straight 3.5% tax to all of their sales ( regardless of profits ) - then these big companies will both increase their pricing by 3.5% or more - and quite likely give out less donations as there is no tax incentive to do so.