One would think. Which would tend to mitigate against their being the culprits. Even though the design of the bomb is consistent with one AQ has advocated before.
Perhaps this is a lone wolf sort of thing--whatever the ideology behind it.
If it is a lone wolf, he (or she) either knows that the intended audience will see the symbolism underlying the attack, or doesn't care and just wants to cause suffering.
The bomb is consistent with what Al Qaeda and the Taliban have used abroad, but is also consistent with the Atlanta Olympics bomb. It's just a very simple bomb design. I don't know that we can read too much into the design of the bomb.