I believe the legal rationale for “suspended with pay” is to remove the defense of a double penalty. In other words, if the employee was suspended without pay, he could argue that he has already been punished and cannot face further discipline. So to avoid that, the employer will suspend with pay and thus will be able to fire or impose a more harsh penalty after the investigation of the particular incident.
I suspect there is some language in the contract that addresses this.
In the real world, employees are often suspended with no pay. And the reason for the suspension in many cases is simply to allow the company to arranged their final pay check, accrued time etc...They'll tell ya you're suspended pending investigation, but most of the time it's CYA until they can pay off the one who's being fired.