While labor laws are indeed relevant, I think prevailing on-call compensation standards for software engineers have more to do with companies applying their own fairness standards to a tight job market than strict legal compliance.

In particular with regards to US law, salaried computer employees (and highly-compensated employees) are 'exempt' from the minimum wage and overtime rules of the Fair Labor Standards Act, so while it's true that you need to pay someone for doing work on an on-call shift, I don't think a US employer is necessarily legally required to pay them any _extra_ if they're an exempt employee.