The argument is that there are certain fixed costs of supply whether you use a little or a lot. Plus the suppliers have faced extra costs in taking on customers from suppliers who have gone bust, installing smart meters and other green levies etc.
But the standing charge isn't really 'a hefty bill', it's a little over £20 a month for both gas and electricity, so a small percentage of the bill for most people. It's the increase in unit cost, especially for gas, that's the problem. This has gone up six/seven or eightfold for many people.
The standing charge is only really an issue if you're a low user through necessity (rather than due to living in a very small, well insulated property and/or being out of the house a lot) so they could help with that by extra grants for people on lower incomes, which they are already doing to a degree with extra winter fuel payments for pensioners and the extra £650 for those on means tested benefits, which will go some way to helping people but, given further increases in costs since these were announced, are no longer enough and need adding to.