Is online poker a scam
It does have risks.
It's illegal in the United States to establish a gambling site on the Internet. It violates the Interstate Commerce Clause.
Because of this, the sites that host poker for real money are housed outside of the United States and as such, are not regulated by the gaming commission.
When one plays poker on a web site, they're essentially trusting the web site to regulate itself, which may or may not be in the interests of that site, and they may or may not have the tools to do so.
There have been instances in the past of the programs being used by web sites being compromised to outside parties who then exploited the program to cheat at the game itself.
These are few and far between however, and any ads that one sees for "how to cheat at poker" should probably be ignored, as these most certainly are scams. If a cheater knew how to cheat, they certainly wouldn't advertise.
It is indeed possible to compromise the sites however, and it's been done in the past.
Beyond that, there are methods to ensuring a greater degree of success at a poker table that can be accomplished on the Internet that include collusion and other behaviors that would get a person thrown out of (if not arrested in) a genuine casino, but that are entirely undetectable through an on-line casino.
It's probably not fair to call real money poker sites "a scam" as some do work very hard to ensure a legitimate gambling environment.
It would also be unreasonable to say that these sites are not at risk for compromise and even in cases where they're not compromised, it is still possible for clever users to manipulate the odds at a poker table in a virtual environment.