It's true. Screenwriting – over all other forms of creative writing – is bound by rules: from format and structure to white space and page count. Screenwriting is visual storytelling, and therefore, the screenwriter must write in present tense: only what the audience can see and hear. The screenwriter must also be clear and concise, yet still creative – both in action description and dialogue. Creative brevity is the screenwriter's steadfast ally and most powerful weapon.
The screenwriter doesn't have time to explore the story through long-winded, soul searching monologues, and the script can't be bogged down with the subtle intricacies of every little detail. There's no time for that, and the screenwriter must be concerned with time - Always! When writing a script, you only have between 90 and 120 minutes to tell your story. That's not a lot of time, so script economy becomes something the screenwriter must strive for. If a scene does not illustrate character or move the story forward, kill it.
BUT... rules are made to be broken. Take Quentin Tarantino, for example. This guy is one of the best rule breakers in the business.
Rule #1 – film is visual, avoid scenes in static locations. (BUT... in Pulp Fiction, Tarantino has his characters sitting at tables, sitting in cars, or standing in hallways with little or no visual movement).
Rule #2 – film is economical, limit dialogue scenes to 3 pages. (BUT... in Inglourious Basterds, the shootout scene at the La Louisianne tavern is a 25 page dialogue scene, spoken almost entirely in German).
Rule #3 – film is action, force your characters to take action, not just talk, talk, talk. (BUT... in Reservoir Dogs, Tarantino's characters talk, talk, talk their way to the truth).
Three great films, one great writer/director, and most definitely, the exception to the rule. Sure, Christopher Nolan breaks traditional structure rules when it comes to complexities of non-linear plot (Memento), but for the most part, when it comes to genre films - which is what MOST FILMS are - rules are writing gold.
Nobody goes to a rom-com to be surprised in the end. In a slasher film, a majority of the audience is actually rooting for the bad guy. In an action film, the audience is waiting for the next cheesy one-liner. The audience has very specific expectations, so it's important to understand the rules of each genre in order to meet their expectations.
People go to see a Tarantino film mostly because, well... he's Tarantino AND his audience has come to expect a certain stylistic quality. Same with Nolan. So if you're going to break rules, make sure you're a famous director with a solid track record, or at the very least do it with drama (e.g. Pulp Fiction, Inglourious Basterds, Memento.)
But if you're a beginning writer - and I stress BEGINNING - the Top Ten Screenplay Essentials are a guideline that will help you to learn the difficult craft of screenwriting... because you have to learn the rules first before you can break them.