Iowa "gambling" law is quite complex.
At the outset it must be noted that Iowa is not a state where the "skill v. chance" argument is applicable, Iowa specifically mentions poker in a number of contexts, and has a general rule against playing ANY game for money:
"Section 725.7 Gaming and betting -- penalty.
1. Except as permitted in chapters 99B [bingo] and 99D [racetracks], a person shall not do any of the following:
a. Participate in a game for any sum of money or other property of any value."
There are exceptions, however, quite a lot of them, in fact as Section 725.15 states
"Sections 725.5 to 725.10 and 725.12 do not apply to a game, activity, ticket, or device when lawfully possessed, used, conducted, or participated in pursuant to chapter 99B, 99F [pari-mutuel horse race wagering], or 99G [licensed riverboat gaming, etc.]"
99B is the big area for exceptions. You can find the whole statute here: http://www.gambling-law-us.com/State-Laws/Iowa/
The cliff notes of that long list is that poker can be played is certain public places provided a license is applied for, a large number of restrictions are met (basically no rake), and no one wins or loses more than $50 in a 24 hour period. You can also play among your friends without a license, but the $50 maximum win/loss rule is still in place. Poker Tournaments are specifically disallowed as "contests of skill." But probably fit the social exception as long as that $50 rule is complied with.
Your specific question has to do with online poker. Iowa has not passed any law that specifically covers online poker. So that is argument number one, given that the other laws clearly only contemplate live play, should they be read so broadly as to include poker played on the internet? Second, an Iowa law on internet play likely violates the Constitution's Commerce Clause (check the main website for progress on the Commerce Clause challenge to Washington's online poker law - GO LEE ROUSSO!). Thirdly, it may well be argued that so long as you dont win or lose more than $50 in 24 hours playing online poker, you have complied with the Iowa "social gaming" exception.
There is no Federal Law against playing online poker (see the the sticky in this forum on Federal law).
However, what you are specifically running into is what we call "over-blocking" and is the devious result of the Unlawful Internet Gambling Enforcement Act of 2006 (the UIGEA). Of course you can find much discussion of the UIGEA on this site. The bottom line for your situation is that under the UIGEA a bank has no liability for blocking a lawful transaction if they can claim any reason to believe it might be related to unlawful internet gambling. On the other hand, banks can face a stiff penalty for facilitating money transfers fro unlawful internet gambling if they do it knowingly, or if they fail to follow the regulations. The regulations have still not been finalized, but some cautious banks have already decided what the "smart" move is under the UIGEA: they simply refuse to do any transactions that can arguably be seen as gambling related.
They really do not lose much business over this, and dont have to worry about whether they are right or wrong regarding the specific transaction.
In sum, it is highly unlikely that your online poker playing is violating Iowa law. That cannot be stated for certain, however, because it is possible an Iowa Court could interpret the law to apply, and the Commerce Clause argument ultimately fail (Courts often do strange things). Because it cannot be stated for certain you internet poker play is legal, and even though it equally cannot be stated for certain that it ISNT legal, the UIGEA gives your bank the absolute right to refuse to process your online poker transactions.
Try and find a less cautious bank; many banks hate policing their customers and simply wont do it unless forced - until and unless the regulations are finalized, no bank is "forced" to do this at present. The most likely banks to not care are the small local banks, but this is not always the case, shop around.
On the bright side, there is no crime you have committed by making these transaction - you have already suffered the worst that can be done to you: your account is closed and your money returned.
Hope that helps, even if the news is not good.
Skallagrim