Libraries are *not* executable, correct?

Why do people set the executable bit in libraries?

I thought they were just ignorant/careless, but I was just reading
some Adobe documentation:

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UNIX:
Before you can compile the samples, you must point the makefiles to
your gcc 3.2 or VisualAge XL 6.0 compiler. Make sure the permissions
on all libraries are set so that the dynamic loader can find and load
the libraries.

chmod o+x libraryname
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I just checked in a Linux server and most *.a libraries do not have
the execute bits set, but most *.so files have it.

What's the story with this?

-Ramon