Ok, So Britney Spears doesn't sing live on stage*. You try dancing around and singing at the same time. It's not easy! I'm winded just thinking about it.
Do we care that she does it? No, because we KNOW she can sing and it's Britney Freaking Spears.
BUT if you need more proof that Britney is in fact a vocalist, just take a listen to this leaked audio of "Toxic" with zero autotune.
*Britney Spears' manager confirmed that she does not singing live at her Las Vegas Piece of Me show. (Source)