From:
http://www.nolo.com/legal-encyclopedia/employment-at-will-de...
"An at-will employee can be fired at any time, for any reason (except for a few illegal reasons, spelled out below). If the employer decides to let you go, that's the end of your job--and you have very limited legal rights to fight your termination."
Now after reading the above sentence, do you still feel weird about this?
How would you feel if a company interviewed you with absolutely no intention of hiring you, even if you did really well, and was just experimenting with their interview process or training interviewers?
A job to you is life changing. To a large enough company, you're just a conversion or churn metric.
b) That happens all the time. For instance, in order to get an H1B, a company has to document that they can't fill a given position in the local market. Which you accomplish by putting out hyper specific job ads.
If you want to be nice about it, don't pull that game on small startups but that's about it.