Wait... You can log in to your own online bridge game and see all the hands? Did anyone expect this to not result in blatant cheating? I'm surprised it's an option at all.
Well, people could already cheat trivially by communicating with their partners, which would change the game just as much as being able to see all the cards at anything above the beginner level. It was already completely reliant on the honour system.
Still, being able to self-kibitz opens an extra avenue of cheating compared to collusion: you can cheat even if your partner is honest.
It is possible to disable kibitzing for a table (or a tournament, or a team match etc.), but kibitzing is a feature. It's often both educational and entertaining to kibitz high profile players and participate in the play commentary and banter.
The downside is that people can kibitz their own table (if kibitzing is allowed) using an alt account. Yes, it's a risk, but there are lots of ways to cheat online. It's trivial to cheat using out of band communication with your partner, as a sibling comment points out, or use a computer program to calculate odds if cheating without the knowledge of your partner.