Printing MSRP on the product happens all the time. Arizona Iced Tea is a prominent example.
Are you sure that they don't? I honestly wouldn't be surprised if Walmart does have clear policies around prices of any kind being printed on packaging. And it wouldn't be a stretch to assume that if they have a policy, it would disallow printing of any price which is less than the highest price Walmart intend to charge at retail.
Meanwhile, I'm merely saying that it wouldn't surprise me if such a policy exists, whether formally or informally. The distinct lack of any product on Walmart shelves which has a below-RRP price printed on its packaging is strong circumstantial evidence at the very least.
The fact that you mentioned is an extremely weak evidence in the light of very thin margins Walmart has (under 3%), which is more than 10 times less than the old store tax, with probably even smaller incentive to combat.