Montagny was cocaine, so not a PED, and the others were not busts by the FIA. I have long suspected that there is PED usage in motorsports and that the FIA would much rather turn a Nelsonian eye to it.
Montagny tested positive after a race, so he may well have intended or imagined that the cocaine would enhance his performance, regardless of whether it actually did. Wrt the others not being FIA busts, they were busts mostly if not entirely by FIA affiliates; I'm not sure how it would have worked if the positive test had been reported but the national affiliate had chosen not to take action, but as a rule one would expect the local authority to act forthwith, precluding the need for the FIA itself to get involved.
Wrt your broader point, you may be right that the FIA would like to turn a blind eye. It is however my sense that Todt's FIA has chosen to take the politically correct route whenever it is possible for it to do so. As almost everyone condemns the use of PEDs (unlike for political disputes in F1 host countries, where things are often ambiguous), for the FIA to come down hard when there is evidence of PED use should be a no-brainer for them.