Walmart easily owes over 3 billion in taxes every year. There is a tiny almost non existent chance they will get a return as profit. However there is a 99.999% chance they get jet.com for free when they write it off as a complete loss in 2 years. If nothing else they get a valuable 3 letter domain name for nothing.
You're misunderstanding how taxes are calculated. If they could deduct the writeoff, they'd make back their marginal tax rate times the purchase price, so they'd still lose around $2B. (Similarly, when people deduct their mortgage interest, they don't get the mortgage "for free".)
