I would like to emphasize that a kashrus symbol is there for people to look at and read.
So one should note if it says OU-D or not and if there is in fact a hashgacha or not.
The same with restaurants, ,you should see if they have the hashgacha and if the certificate is up to date. I make it a point to check and have at times seen out of date certificates. I have walked out of establishments that only had an old certificate and could not demonstrate that it was renewed.
However, in the case of the candies first referenced to, it seems even the OU recognized that it could be confusing based on their response. One would not suspect that they should have to look for multiple hashgochas on one packages (unless you are looking for the added sticker)