Becasue that amp is designed to work in a 2.0 configuration. That is, it was designed to work with 2 full range speakers of a resonable size. Using a sub with it kind of makes cambridges efforts at perfect sound reproduction from a single amp a little pointless. Amps that are designed to work with subs have a special 'sub out', which is a single channel line level output. This output can be adjusted on the amp to redirect all the lower frequencies to the sub only (leaving your L/R speakers to get on with re-producing a clearer mid/hi frequency).
Like I said before, if you have the sub linked up to the record output of the amp, then the volumn of the sub wont go up/down with the main volumn control, which is not very useful as the sub level cannot them be matched. If you have the sub connected to the spare speaker output, you need to make sure the sub is set to simulate a 4 to 8 hom load, else it will damage the amp. Using a sub off the speaker output is a very low quality way of doing it, as it amplifys all the amp hiss/thd etc (thats why it is avoided in car installations as well).
I personally would not sell someone an amp that is designed to work in a 2.0 configuration a sub woofer to go with it; the 2.0 amp paired with the correct speakers will produce a better quality of sound without a sub (as it was ment to do). Sub woofers (besides from in-car installations, as the component speakers in a car are not capable of LFE in door enclosures), are normally used in AV (audio video) setups. In a DD5.1 or DTS signal (DD5.1 is the main soundtrack format on 99% of DVD/BD's), there is a dedicated channel/track for the LFE (low frequency emmitter), which is normally the sub. If your not using a signal with a '.1' audio track, then post processing needs to be done to split the signal (can be done with a crossover).
Hope this helps, I have tried to keep it simple, so let me know if I need to go into more detail