If MA is not taxable, do you include it anywhere in the Self Assessment? They are pretty much the same thing, I can understand why one is and one isn't?
Maternity Pay is not taxable as you say, it's one of those benefits that are on HMRC list of non taxable (see EIM76100 for the full list). I don't think it needs to be entered on the tax return.
I think it's just a case of they haven't got around to it yet. SMP was easier to tax as it is administered through the PAYE system. As you say, they're pretty much the same thing except the self employed get MA. It's therefore quite unjust to tax one and not the other. I guess the objections have not been loud enough yet, but I'd hazard a guess that if they were forced into a change, they'd both become taxable
