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

Hi Robert -

It is a combination of factors ... right?  

1) Speaker sensitivity for sure.  2) The amplifier's continuous power output per channel measured across the full frequency range (WPC, 20 - 20,000 hz).  3) The size of your listening space.  4) How loud you like to listen or better said, are allowed to listen (apartment house vs private house, family complaints).  5) Minimum vs Maximum Power.  You should have more amplifier power than you really need as you don't want to max out or floor your amp.  Need clean power.   

It is not a tower vs bookshelf thing, per se.  It's how all these factors come together.  I assume that this is for music and not home theater, as HT really eats up power.  

Sales rep did not know his stuff or is compensated to direct sales in a certain way.

Best, Rich 

"People" say all kinds of goofy things like 
"The earth is flat". 
Questioning their reasons is futile and   
Frankly I don't care.

The amp and speakers are compatible and will make sound and he wants to sell stuff so put 2+2 together. That could be the extent of his expertise.  Most customers there would never think twice about it. 

It is either ignorance or the salesperson pushing a specific sale or a simplification of what is actually a little bit more complicated.  Smaller speakers might tend to be more inefficient and they also tend to be not as capable of delivering very high volume level.  A lower powered amp will reinforce the notion that they system should not be played as loudly as one might play a bigger speaker.  I know it is a common belief that more speakers are destroyed by pushing a lower powered amp into clipping, but, ask most speaker manufacturers and they will tell you that is not the case--most speakers are damaged by simply being overdriven (even if the signal is clean). 

Definitely an ignorant comment or as Larry says a salesperson with an agenda. And my eyes must be playing tricks I thought I saw a post by @viridian talk about pleasant surprises!