Question: I recently got my US visa and I am really happy about it. Also, I recently saw a documentary where a US immigration officer was denying entry to some people. I never thought your entry could be denied if your visa was valid.

What are common reasons for US immigration officers to deny entry in the country?

Answer: 
Any visa is never an automatic permission to enter the USA, and that’s clearly stated in many places during the application.

The core reasons to get rejected are that the officer has the impression that you are either

* going to violate the limits of the visa,
* lied in the application, or
* have a criminal intent.

The details depend on the visa—with a tourist visa, if it seems you are planning to work it is a problem—and it does seem that you are planning to work if you don’t seem to have enough money.

Most visas also have a limit for the duration of your visit, and if you don't have a return ticket, or only a vague idea where you will go, it looks fishy, etc.
A visa is just something that you need to enter the US. The visa does not indicate any decision to let you enter. It just allows you to be present for a decision to be made!

The visa is needed so that someone can make the decision: it's a necessary but insufficient part of the process. The actual decision is made each and every time by at the CBP post where you enter the US. Typical reasons for denying entry include:

* attempt to contravene immigration laws, e.g. applying for a tourist stay and then unauthorized employment, or giving reasons to suspect that such may be attempted in the future,
* prior actions in contravention of immigration laws, e.g. overstaying the entry permit duration without extension or advising someone how to "cheat" on a visa application; In other words: consequences of helping others skirt/break the law are at least as bad as doing it yourself is,
* prior criminal charges and/or judgments, whether in the US territories or outside,
* determination that there are grounds to believe that the entry of the individual is contrary to the interests of the US (or sometimes also its allies),

etc.

Note that the initial authorized duration of stay is tangential to any dates on the visa. It has everything to do with the decision made at the port of entry. The CBP officer will write in your passport and/or on your entry card how long you're authorized to stay. This may be shorter than what you expect in certain cases. E.g. just because the visa allows a stays of a certain duration, that's the upper limit. The actual length of stay authorized is determined on a case-by-case basis each time one enters the country, and an authorization may be denied as well of course. Say, a B1/B2 visa has a maximum length of stay of 6 months. I've seen people get permission to stay for 2-4 weeks if it was determined that that's all they need for their scheduled activity within the country (say a conference/symposium). At the same time, lying about what you want to do is just as bad and is grounds for refusal of entry, so don't try to "pad things". If you don't know how long it may take, or if you have potential extensions to your plans, just say so. You may be asked to provide some documentation that you have the means to afford your stay, or are otherwise financially supported, in any case.

And the flipside: just because someone's visa is expired doesn't mean they are not authorized to stay in the US. An immigration status may be adjusted without any physical visa being added to the passport, but other documents will substantiate such claims, e.g. a decision to extend or to adjust an immigration status. Such decisions are of course not awarded willy-nilly - there are copious regulations around that.