consulting-firm = pimp

I fully agree! Consequently we are the whores. Underpaid, lousy insurance, bad working hours etc. etc.

Most software consulting firms in Mexico play a good number of tricks to maximize the profits

This is universal corporate behavior. It's called capitalism;)

"Are you available to work overtime?"

In my experience this is normally phrased like JavaFan did. Belief it or not, during the intake/interview you also have power. It's not one-way traffic. I normally respond something like, "of course I'm more than willing to do overtime", "it comes with the job" etc. But, and this is important, you add that this does not mean doing overtime STRUCTURALLY without getting paid. Of course you have to phrase it politically correct. My contracts always had a specific clause on doing overtime.

It has become fashionable to impose absurd and doubtfully legal contract clauses

In my experience this is more the rule than the exception. But there is a big difference between putting something in a contract and being legal. They might violate laws themselves, it is worthwhile making inquiries. To give one example, a friend was not allowed to transfer to the customer and do the "same kind of work". It turned out that just changing his job title was sufficient to avoid any legal problems. Keep in mind that clients are not too happy when their employees get sewed. Normally the threat of not hiring people from the consulting company anymore settles the issue;)

You can always change pimp!