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

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.   

That is too broad a statement 
my Elac Vela 403 speakers are 87 db efficient and are a 4 ohm load  they need a little extra  power to get them working optimal
.the-true  efficiency of speaker,not just rated  ,many companies fudge the actual  efficiency. Unfortunately there is no standard how to test from true 20 khz to 20 Hz sweep for a specific time and spl level would be good if all companies did this, and  on bass at least go down to advertised specs,for a extended period ,and how easy a load is the Xover ? these are 2 key factors  how a amplifier will respond . Sometimes getting  the synergy is a lot of trial and error .

I have found many of the Magnolia sales people to not know what they are talking about.  This is why I would prefer to talk to a good independent audio dealer.

people say stupid stuff because either they are stupid or they think you are