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


I was in a BB Magnolia recently and they had a McIntosh MHA150 integrated headphone amp that can also do 50 watts per channel to speakers. The sales rep said it "should be good for small bookshelves but its not enough power for towers". 
I've never understood this line of reasoning.  Towers are typically more sensitive than bookshelves. Is there an actual reason a small amp like this couldn't do just fine for towers that are equally or more sensitive than similar bookshelves?
roberthz
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.