Yes the reason why Denon included those particular pre-outs is because it's their budget AVR and they don't include 7 amplifiers, only 5.
It would probably cost them as a manufacturer less than $5 to add all the other channel pre-outs, but they really want you to buy a more expensive model in order to use separate amps for the main surround channels.
Normally the only option with that type of receiver is to use a high-to-low level converter, which degrades fidelity. If it was strictly a 2-channel system and you had an integrated amp (with its own volume control), the receiver could be used as a basic source switcher by utilizing the VCR/Tape pre-outs but that's about it.