No, it's not, it's an anti-subsidy, because money is taken away from workers for getting paid by an employer, increasing the amount an employer needs to pay.
> because Wal-Mart is not having to pay their workers a living wage
WalMart wouldn't have to pay a living wage without it, either. Public assistance programs (aside from work requirements and the cost of lost benefits from failure to comply) actually increase market clearing wages, they don't decrease them.
> because they can simply tell their workers to sign up for public assistance
The people involved can sign up for public assistance with or without working for WalMart; further, they lose benefits due to mean testing based on outside income, which means WalMart has to give them >$1 of take home pay to get labor they would be willing to trade for $1 of net income increase.
That's not a labor subsidy, and if it was the simple fix would be to make employed people categorically ineligible for federal benefits—we don't do that, because we know it's not a wage subsidy, and all the hyperventilating about it being a wage subsidy is (at least on the part of public officials and other people that know anything about policy) dishonest grandstanding.
(Now, work requirements in benefit programs do make them wage subsidies, because then the threat is “work at whatever job will have you or lose public benefits in addition to the marginal benefit of pay”; that's easily solved by removing such requirements.)