The creation of the Authority is in all respects for the benefit of the people of Illinois and for the improvement of their health, safety, welfare, comfort and security, and its purposes are public purposes. In consideration thereof, the notes and bonds of the Authority issued pursuant to this Act and the income therefrom shall be free from all taxation by the State or its political subdivisions, except for estate, transfer and inheritance taxes. For purposes of Section 250 of the Illinois Income Tax Act, the exemption of the income from bonds issued by the Authority shall terminate after all of the bonds have been paid. The amount of such income that shall be added and then subtracted on the Illinois income tax return of a taxpayer, pursuant to Section 203 of the Illinois Income Tax Act, from federal adjusted gross income or federal taxable income in computing Illinois base income shall be the interest net of any bond premium amortization. The exemptions of taxation provided by the preceding sentences shall not apply to the income on any notes or bonds, a major portion (determined in the sole judgment of the Authority) of the proceeds of which are to be used to finance housing related commercial facilities.