Cogito-
Because it's out of character for Labour to propose tax-cuts and, as Claig points out, far more typical of Conservative economics. If Osborne was to announce anything like this in a budget the Labour front bench would normally be waving their papers, shouting him down for looking after the wealthy or bribing voters. They know that approach wouldn't go down well this time around. If Osborne now announces anything similar in the budget, Balls will say 'it was our idea first' and, if Osborne's tax cuts are lower, Balls will say 'we'd have done more'.
So the conclusion is that Balls knows there will be tax-cuts in the budget and is trying to steal Osborne's thunder. Pure politics.
Your perception is obviously based on a strawman. Cutting taxes isn't necessarily good or bad, or regressive or progressive. It's a matter of how they are done.
Of course the Tory scum would typically be criticised for cutting taxes, because they would tend to cut taxes for the wealthiest, and they would do so in the name of 'trickle-down' economics; that idea which started in the 1980s, and which has shown, not only to be false, but to have precipitated a massive rise in wealth inequality.
Cutting taxes for the poorest is not a Tory idea. It's not a right-wing idea. It's a Keynesian idea.
It's Keynesian because it concentrates on stimulating demand during a recession, and it concentrates on the demand, rather than the supply side. That is to say, rather than trying to make it easier for jobs to do business in the UK (this is not a problem, the UK already being relatively deregulated and low-tax environment), one tries to put more money into people's pockets, so that they can spend and get the economy going again.