State and local election officials across the United States have a practice of releasing vote tallies as soon as they are available and many jurisdictions invite representatives from the news media and other interested parties to observe the vote-counting process. For example, Arizona Title 16, Section 622 states:
At any time following the close of the polls, except as provided in
section 16-551, subsection C, unofficial returns may be released
during the counting of the ballots by vote tabulating equipment, and
upon completion of the count the unofficial results shall be open to
Most modern voting jurisdictions in the United States post their unofficial results online immediately. Local results are also forwarded to the state election authority, which also generally posts the aggregated totals as cities and counties submit their results. In the modern era, it would be difficult, if not impossible, to keep results from being disseminated via email or social media, let alone traditional television and online news outlets.
There are no controlling federal regulations on this issue, though issuing projections about the outcome of the presidential election has been an issue since the 1960s, according to this article in the Yale Law and Policy Review from 1984.
One of the noteworthy complaints was that NBC's early projections of a Ronald Reagan landslide -- based on exit polling -- in the 1980 election discouraged voters who would otherwise have vote for Jimmy Carter.
After pressure from Congress, in 1985 the three major networks (NBC, ABC, and CBS) agreed not to characterize election results based on exit polling before the polls were closed.
In 1992, more than 150 members of Congress signed a letter to the presidents of the three networks asking them not to project a winner until polls had closed in the western states (Washington Post). CBS vice president Joe Peyronnin is quoted as saying it would not be appropriate for the networks to withhold the results of electoral college tallies based on publicly released information and suggested Congress could remedy the matter by setting a uniform time for poll closings.
In 1990, ABC, CBS, NBC, CNN, Fox, the Associated Press and the Republican and Democratic parties formed the Voter News Service to consolidate their exit polling efforts (Wikipedia). In the 2000 election, the networks that used the results of this exit polling consortium to make the early and flawed call of Al Gore winning Florida and delayed calls of George Bush winning other states drew criticism about potential voter discouragement -- notably from Republicans. The consortium was disbanded in 2003 following similar complaints.