Sadly, it is natural. That's why Greta would be barred from serving on the jury, if her friend had injured someone; her natural bias would preclude her.
Not long back, my local paper reported that a man very local to me had been convicted of raping young teenagers and threatening them to keep silent. Also up in the dock were some female teens around Greta's age. They were friends of the defendant, and so entirely subscribed to his version of events. As a result, they'd chosen to engage in a coordinated campaign of harassment against the girls he'd raped. One girl received a suspended sentence, and as she wasn't in prison, she spent the evening defaming the victims in the newspaper's online comment section.
The ability to look at one's friends/family objectively/semi-objectively and say "no, you're in the wrong" is a skill mostly only developed in full adulthood, and often, not even then.