There is absolutely no reason to sign a training contract. No other airline makes you sign one.
Mesa management knows that when you get there, you will realize that in addition to the exceedingly low pay and no retention bonus as offered by the other airlines, the upgrade ship has sailed.
So all they have to keep you there is the fact that they will sue you to collect the $12,000 from you.
Make your own choices, but be informed. They are going after ex-employees to collect the $12,000.