I don't think there's anything wrong with this, but why would Amazon, Walmart and others not make this type of linking against their terms of service?

It goes against the spirit of affiliate programs, which is an exchange of actual promotion in exchange for commission. The stores aren't gaining anything by allowing it since those links were already published. And it hurts other affiliates, not only by directly taking their commissions (because the most recent affiliate click before purchase counts), but also because it increases the total amount that the stores are paying out.

If this catches on, it will force merchants to lower their commission rates, causing them to lose publishers. I think that most merchants will end up forbidding the practice, just as many have forbid bidding against them in PPC campaigns.