When Doing Something Doesn't Mean You'd Do Something

Megan McArdle:

I get paid for speaking sometimes.  You cannot assume that because I speak to my alumni groups for free, I must therefore be willing to speak whether or not there is money involved.

You mustn't assume that.  You might assume that because she did something in the past she might do in the future.  Which is to say, you might assume that she speaks sometimes for free because she once spoke for free.  You might also wonder why anyone would pay her. The whole thing is highlarryious because each paragraph makes as much sense as these sentences.