While there may be something to your concern, the example you give confuses the idea of how taxes work for companies.

Reddit is already getting a "massive write-off" for its developers salaries. It's called a payroll expense and it's directly deducted from any income the corporation makes before calculating it's profit.

Formula: Reddit's revenue - all of its expense (including payroll) = profit that it has to pay taxes on.

In that same way, corporations that are paying IBM for their consulting services already deduct those payments from their revenue. The payment doesn't need to be a donation. It's a legitimate business expense.