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

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!

I've seen a number of online reviews in which the reviewer states something like "the amp worked well on my small monitors, but it probably wouldn't have enough power for large tower speakers."  Total ignorance of the basics of speaker sensitivity.   

The only reason I can think of for saying low power amps work well with bookshelf speakers is low power amps tend to be small and light enough to fit on the bookshelf.