Actually, it's not.  It's just a misleading pro-M97 talking point.

Only C corporations file their taxes under the corporate tax code.  That particular Anderson Economic Group study addresses 11 types of taxes that ALL businesses (not just C corporations) pay (e.g., property taxes, sales taxes, individual income taxes on pass-through income, motor vehicle taxes, etc)  Corporate taxes are just one of the included taxes.  'Business' tax burdens are NOT the same thing as 'corporate' tax burdens.

Oregon’s business tax burden is the lowest in the country according to this AEG report, not because corporate taxes are low, but rather, because we don't have a sales tax and our relatively low property taxes.  The report itself even says:  “Oregon has no general sales tax, but otherwise is not among the ten lowest-burden states in any particular tax category.”
If you exclude sales tax revenue - an apples-to-apples comparison would put Oregon's corporate taxes around 37th of all 50 states.