Sunny,
I think that the point you might be missing is that the state owns over 99% of the bank. So it doesn't matter what it does with the preference shares. The taxpayer is paying the dividend to itself. It could put the rate up to 20% or it could tear up the preference shares - the taxpayers' position does not change.
Look at it another way
Let's say that the AIB ordinary shares are currently worth €10 billion - the state's stake is €12.76 billion when you add in the €2.76 of preference shares.
Now that the preference shares are cancelled, AIB is worth €12.76 billion.
The state's position has not changed.
Brendan