Again, making 3% on a credit card transaction (less their costs in processing, rewards and other expenses) and interest of varying rates won't cover 25% discounts on millions of dollars in purchases. The other big difference here is that previously the offer ended when a certain number of transactions had been processed. This one has been open for a week and the level of abuse is literally staggering.
I'm not a financial analyst, but from my mathematical intuition I highly doubt the "abuse" can even cause a "loss" that they would bother to put data engineers to investigate. I also would imagine this whole thing falls under the budget of "marketing promotions", which they know they need to bleed money to compete with the many other more popular alternatives.
They are Visa, couple million is their pocket change.