Most of it is legal, and some of it is taxes.
Poker players are not treated fairly when paying taxes. Some states, like CT, do not allow them to deduct losses. Because of this there are cases of poker players owing more in taxes than they actually won.
Legally, as with taxes, poker players have to deal with federal and state laws. These are outdated, difficult to interpret, and change significantly from state to state and from Attorney General to Attorney General. Many laws were written before the internet was commonplace. Some states outlaw betting on all contests except ones "based on skill" or "where skill predominates", so then you have some AGs claiming that poker is not skill based, and some that don't care. There are various rulings that support poker being skill based, and others that rule oppositely (I bet you can guess my opinion on this).
Other states have no laws regarding poker but have attorney generals that claim it is illegal to play online (CT is another example of this). One or two states outlaw online poker explicitly (Washing State). However, to date no one has ever been prosecuted for playing poker online, in any state, ever.
The US has also lost a World Trade Organization ruling due to its anticompetitive stance on online poker and gambling. The US was forced to either pay reparations to Antigua or change its laws. We chose to to not change our laws and not pay reparations to Antigua. So, now Antigua is legally allowed by international law to violate US copyright for profit, up to the amount of the reparations (I think that's hilarious).
The Unlawful Internet Gambling Enforcement Act will come into full force in December. It basically states that banks are not allowed to transfer money to an entity involved with unlawful internet gambling. However, it does't define what "unlawful internet gambling" is, and leaves it up to the banks to decide.
Right now you can bet on horses, buy lotto tickets, and bet on fantasy sports online at state-run websites. You can also bet on "skill" games at sites like worldwinner (based in Newton Mass.) But, it's much harder to transfer money to poker sites because of scared payment operators and over zealous bank enforcement. Once you get money to an online site, you are not up against the casual American player, instead you are faced with people who figured out all the hoops to jump through. The games are much harder to beat now, so it's not worth the time to me.