Raza Panjwani, writing for Recode:

So what are the lessons to take away from this story? First, livestreaming apps have many more applications than just restreaming copyrighted works. Our first reaction to every new technological platform shouldn’t be “but what about copyright?” Second, livestreaming an event that you’re witnessing is different from merely pointing your camera at the output of someone else’s camera. You may still get in trouble (e.g., for violating venue rules, or violating someone’s privacy interests), but it won’t necessarily be due to copyright. Third, protecting the value of a copyrighted work doesn’t require expanding copyright law — sometimes other parts of law, like property law and contract law, suffice.

As a practical matter, these live video apps aren’t a threat to live entertainment or the value of high-quality video broadcasts of the event — they’re not a substitute for the live experience or a high-quality broadcast. Even if camera sensors and wireless bandwidth improve enough to allow us to livestream in high definition, we already have the legal tools, such as the DMCA’s notice-and-takedown provisions, in place to handle it.

In short: No, you can’t. Legally.

Pointing your phone at a TV displaying a game or event is considered copyright infringement, because you’d be rebroadcasting someone else’s copyrighted work (a TV network’s broadcast of the game). Live-streaming from at a sporting event doesn’t infringe on any copyrights, because sports aren’t a creative work, but it could violate some fine print on the back of your ticket. So the question becomes, do leagues, teams and networks want to enforce this? Maybe. HBO and Showtime absolutely have an interest in going after people who live-stream a fight, especially one that costs money to view. You can argue that Periscope provides a real alternative to, you know, paying for the fight. Other sporting events are a little different because it’d be hard to argue that, say, a Phillies fan who wanted to watch Phillies games would just not get cable or MLB.tv and instead rely upon generous Periscopers to provide a broadcast of each and every game. Infringing? Yes. But it’d be a waste of time for TV networks to go after anyone except for the most egregious offenders (people who live-stream games in their entirety).

Leagues and teams going after an account that streams from at a sporting event seems like genuine waste of time. These broadcasts will never (at least not for some time) provide a viable alternative to an actual broadcast of the game, and often, I’d argue, would add to the official broadcast by showing different views of a great play, crazy moment, or genuine news event (celebrations, fights, etc.).

A third thing, not discussed by Raza, is Fair Use. You are allowed to use copyrighted works for commentary, criticism, humor, etc. For instance, streaming or re-posting a small portion of a live TV broadcast for the purposes of critiquing its production or commentators, or for a newsworthy purpose (guy gets assaulted at game), would typically be protected by Fair Use. I’m assuming this is why the Daily Show can pile on CNN and FOX News without threat of legal action– because they’re critiquing, commenting on the work. What’s more is that though leagues, teams and networks may know a particular use is not infringing on any copyright, they may still threaten or pursue legal action… in which case the accused can actually hold the accuser liable for misrepresenting their claim.

Anyway, long story short is that unless you’re genuinely trying to provide an alternative to a broadcast (like live-streaming a Mayweather-Pacquiao fight), you’re probably going to be fine. Especially if you’re at a game. For baseball, specifically, commissioner Rob Manfred said as much in response to (apparently incorrect) reports about the league cracking down on streaming.