Legal online betting in USA
This betting type is very popular for US players. Some of the biggest world tracks are from US-venues. Clients from over 35 states are allowed place bets through the Internet. There are some restrictions by states to which types of wagering you're able to use.
Actually it's legal in 38 states (in over 75% of the USA it's allowed by law). The main reason to take advantage of this betting comes from the Interstate Horse Racing Act of 1978 (IHA). The update in 2000 makes it legal officially for Internet sites. In the 2006 Unlawful Internet Enforcement Gaming Act, it kept off-track betting legal (but only for US-based OTB sites).
However, the horse bet is rich of options, and this leads to different restrictions by state. In some locations it's possible to use all wagering features, in others, you can bet on a single race and no access to players' pools and others.
You can find the right information on the website you choose to gamble online. Where it's not allowed to bet on horse races? That's the list of all states where it's prohibited: Alaska, Arizona, Georgia, Hawaii, Mississippi, Missouri, Nebraska, Utah and some others.
Keep in mind that if you reside in some other state where it's OK to bet on horse races, you still should choose from US-regulated bookies.