that is the inherit nature of android and ios, for that matter. android doesnt really do mutitasking dispite what the internet says. the multitasking is merely a pause of what you were doing. ie it doesnt use cpu on something running it the background unless that app was designed to do so.
sometimes android doesnt even keep the pauses of the things you were looking at eg, if you minimise the browser and play a game, if android needs more resources, itll dump the pause of the web browser from the ram and when you go back to it it has to reload the tabs.
the industry considers multitasking as "the user being able to switch between tasks" where as we consider multitasking to be "the machine can do multiple tasks at once".
the only device that does play browser audio in the background and multitask properly is the bb playbook.
although for android there are apps which do play audio in the background like tune in radio or sonys music unlimited but I havent seen anything for youtube.
