I have been noticing them in my area too! I think it falls into two categories, they really are new inexperienced drivers and want others to know or they are the parents/car owners and they forget to take the magnet off after their child drove.
Personally, I like it. I think it is good to know when someone has less experience driving, the average person can extend some empathy knowing they were like that once. I just hope people aren't using them as an excuse to be shitty drivers.
Side note, in Japan people are required to have a new driver tag on their car. Though I think it is for a different reason then just letting people know their inexperience.