I suppose Samsung tuned the system to avoid false negatives, which might be difficult in all the lighting conditions that might come up in real world use. It's a device for the mass market and average user after all.
Might be that they did a bad job with the Iris recognition, but why not give them the benefit of the doubt and consider that they were aware of the trade-offs involved?
Knowingly and willingly giving users a false sense of security? How is that not worse in every way?
If they couldn't manage to get false negatives down to an sensible level without compromising security in such a blatant way, there's two courses of action: Live with it, or don't release it.
How is this any different than the swipe to unlock pattern? If you go to a starbucks and sit down for 20 minutes in a higher seat, you can get the passwords to probably 2-5 phones depending on everyone's security and how crowded it is (I inadvertently memorized 3+ coworkers' unlock patterns because they're so easy to see).
If someone has physical access to the user they can probably get into most people's phones (that's just saying people aren't careful enough though).
Might be that they did a bad job with the Iris recognition, but why not give them the benefit of the doubt and consider that they were aware of the trade-offs involved?