If the B sample comes back negative, the athlete is cleared, so nothing UCI can gain by waiting really. He's free, even if it goes to CAS him and his Federation would already know anyway.
The A sample is 60ml and the sample rigorously tested for all banned substances. The lab doesn't know what they are looking for, so this sample is twice that of the B sample so there's enough urine to work with.
The B sample is 30ml and only tested for what was found positive in the A sample. i.e. it can only be used to confirm the doping violation found in the A sample, nothing more.
Therefore in Cardoso's EPO found in the A sample, no lab would be then looking for steroids in the B sample, so even if B sample was tested by Châtenay-Malabry lab, they would only be testing for EPO anyway even if contaminated with steroids by them.