I understand your reasoning but the employee would submit his receipt in the reimbursement plan. One of my administrative duties is reimbursements and it took a lot of back-and-forth to get one person seeking reimbursement to submit bills or receipts or invoices and never the credit card statement.
The credit card statement has way too much personal information on it that's none of the employer's business. The only way for the employer to count this as income is to require the employee to submit the credit card statement for review to see if rebates were applied, a further complication being that the rebate would show up on a subsequent credit card statement.
That would be an outrageous violation of privacy and a massive administrative burden to the employer.
The employer is not supposed to know or care if the reimburseable employee business expense was paid by cash, check, credit, or debit card.
There's simply no way for the employer to provide the employee the check stub or information return treating the rebate as earned income, which I assume you're thinking would be subject to Social Security taxes.
I've gotten rebates in the form of checks or gift cards, which are temporary debit cards. None of that would even appear on the credit card statement.
I disagree with you. This is not earned income from the employer to the employee.