The positive regulator is biased up by 0.7 volts to compensate for the series diode in the output. That diode is there to protect the regulator from situations where the output may be maintained after the supply is switched off, for example when a large electrolytic capacitor is placed across the output.
The capacitor question is answered if you look carefully at the way the circuit is arranged, the positive regulators bridge rectifier negative output is 'ground' however the negative regulators bridge rectifiers negative output is the circuit negative output, and obviously cannot be also grounded, as the regulator being used is a 78xx and its output is at ground potential.