RSU's are not reported as "base salary" because they're not base salary. RSUs are bonuses, and cannot be "base salary" by definition - since they're restricted. RSU's are generally awarded as a retention award, and vest during the period of the time the company is interested in your staying. Similarly, any other discretionary or conditional bonus cannot be and is not considered as "base salary".
The base salary on the visa application must match the base salary on your payslip. As such, this is easy to confirm and will be checked during DOL audits, when the employee applies for a new sticker, an extension or a green card. I don't think anyone would want to play with this - this is the easiest thing to catch and any VO at any consulate will ask for your recent payslips any time you come by them.
In another thread you mentioned that the IRS treatment of RSU vest as salary confused you. The IRS treats any compensation as salary, and that includes RSUs, bonuses, and any other reward you're getting in return to your personal services. But "salary" is not the same as "base salary".