Why is it legal for some job positions not to have to pay overtime while others need it paid out. IT employees fall in this category (not needing to be paid). It translates to false productivity and suppresses employment (why hire someone when Jane has to work extra hours for free). If overtime could be billed at 1.5x it would be cheaper to hire someone else.
The literal difference is that most IT employees are "Exempt" and employees in many other fields (construction, retail, etc.) are "Nonexempt" under the classification by the Fair Labor Standards Act. A FAQ from the Depart of Labor on the FLSA:
To continue the hijack, this is largely the case because IT employees have been loathe to unionize or organize in any way. For many reasons, those in all parts of the computer industry have avoided organizing their efforts which manifests with a pretty unfair power balance between employees and employers.