If a photon with less energy than a band gap hits the band gap, why will the material be transparent? Although the photon cannot raise an electron in the conduction band to the valence band, why isn't the photon absorbed and turned into heat?
The answer is contained in the question: in order to be absorbed the photon needs to raise an electron from the valence band to the conduction band.
Absorption here means transmitting the photon energy and momentum to the semiconductor. Being an electromagnetic wave, the photon is coupled to the charge in the semiconductor, and the only possible change of the charge state is moving an electron to a different energy state. This is not a problem in metals, where the conduction band is only partially filled and electrons can be excited to any energy within this band, which is why metals are reflecting in the visual spectrum.