Google, which owns YouTube, makes its money through advertising. It provides 'free' video content for you to enjoy, and you return the favour by viewing some ads (or by paying for a YouTube Red subscription, assuming it is available in your country). Download the YouTube video to watch it offline and you're no longer keeping up your part of the deal, so why should it want to scratch your back?
It's pretty obvious that Google would want to avoid you downloading its content. Not only does it want to protect its earnings potential, but it has to protect the content creators who upload video to its site who may also be making a living from that content. And this is why you won't find apps that allow you to download YouTube in the Google Play store: Google does not condone such a practice.
But is it actually illegal to download YouTube?
For personal use, no it is not illegal to download YouTube video. But it is immoral.
Taking steps to avoid video-streaming quality issues and overstepping your data-downloading limit are common sense, and avoiding advertising is not illegal (ad-blockers are the bane of our industry, too).
In-browser ad blockers are legal - if a little unfair to the publisher when accessing free, ad-supported media - and no-one will pull you in front of a judge for leaving the room to make a cup of tea during a TV ad break. You can even skip the ads after a few seconds on most free catch-up TV sites, and fast-forward the ads on recorded programmes.
But consider the fact that lots of the people who create YouTube videos rely on the money they make from the adverts displayed on their clips. If not illegal, we can surely all agree that this is morally wrong.
Downloading video from YouTube does breach Google's terms of service. Within Section 5.1 it states: "YouTube hereby grants you permission to access and use the Service, subject to the following express conditions, and you agree that your failure to adhere to any of these conditions shall constitute a breach of these Terms on your part: you agree not to access Content through any technology or means other than the video playback pages of the Website itself, the YouTube Player, or such other means YouTube may explicitly designate for this purpose.
"You agree not to access Content for any reason other than your personal, non-commercial use solely as intended through and permitted by the normal functionality of the Service, and solely for Streaming. "Streaming" means a contemporaneous digital transmission of the material by YouTube via the Internet to a user operated Internet enabled device in such a manner that the data is intended for real-time viewing and not intended to be downloaded (either permanently or temporarily), copied, stored, or redistributed by the user.
"You shall not copy, reproduce, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any Content for any other purposes without the prior written consent of YouTube or the respective licensors of the Content."
In other words, you are permitted to view YouTube video only through Google's own website and apps. You are not permitted to view YouTube video offline on your iPad, iPhone or Android device, PC or laptop through a third-party app.
Such third-party apps will continue to pop up on the web, and people will continue to use them to make available offline their favourite YouTube videos. Google might not be aware that you're using them to download content from YouTube, and even if it did any threats of banning you from the service are all but impossible to uphold. After all, you don't need to be signed into your Google account to access YouTube.
So, at the end of the day, it's really up to you to decide whether or not downloading video from YouTube for personal use is something you should be doing.
Piracy and copyright protection
A major concern with downloading YouTube video and using it in your own projects or for your own gain is the ease with which you can fall foul of copyright protection.
You might think it's okay to download commercially available music or video for free, but media companies are not in the business of giving away their goods for free - and rightly so. The more people who follow your thinking, the less money they make, and the less money is awarded to the original content creators - often ordinary people struggling to make ends meet.
Google is hot on piracy. It encourages users to draw its attention to any videos that breach copyright and, where a video uses a soundtrack or other element owned by another person or company, it will award the original content creator any credit that arises from it.
We won't go into depth over the ins and outs of online piracy here, suffice to say that in order to stay on the right side of the law, any content you do download from the web, for which you do not own the copyright, must be for your personal use only. You must not distribute or financially benefit from that content. It must also come with the express permission of the copyright owner.