Why do people say low power amps should be good for bookshelf speakers?

The only reason might be to eliminate over exuberant use of volume, which could damage speakers, so there can be no come back on the retailer.

Maximum power is often quoted when THD+N reaches 1%. In my mind that's a dreadful figure.

Therefore maximum power is probably best considered at half the advertised figure. Just don't screw the volume up to full.

You are correct there are a lot of big speakers that would thrive on fifty watts per channel and there are a lot of bookshelf speakers that would too because they do not handle much power but everything is about mating the right amp with the right speaker so it is best to try the amp with the speaker that you like and see how the combination works in your room with your music with the rest of your system.

I've basically got the same amp, the MHA100 and it's a REALLY NICE sounding set up through the speaker taps. I'd say 90db or better for a larger room or loud volumes....bookshelf or tower. My 95db bookshelf speakers never make the amp break over 2 watts even at spirited volumes. Enjoy. 

Salesperson speaking here...Noromance & aewarren are on the right track.  The BB SP was making an assertion that contained an assumption, which is not great critical thinking, but the gist of it is that people who want big sound in big rooms should buy big speakers and amps to match...and the converse is also true.

It depends.  Many bookshelf speakers are not sensitive but also don't have deep bass response.  When SPLs go up to high levels (and power draw goes high), usually it is at the lowest frequencies (below 40hz).  You will rarely see frequencies above 60hz rise more than 10dB above average volume levels even in the most dynamic music.  

I have speakers here with a frequency response that begins rolling off at ~60hz and 45w is plenty to drive them at reference levels despite having an SPL of 84dB.  If they started rolling off at 40hz, I would be far more concerned and would seek a lot more power.