Ultimately I think if it's her choice to parent that way, then it's her choice regardless of how you, I or anyone else feels.
My DS1 wasn't interested in telly until he was probably 25 months or so. And now if I let him, he'd watch ABC2 all day. In fact, he's a bit addicted to Play School. My 16month old is like your son, likes a little dance to the music and that's it. I do use the tv to give me some "mummy time", so depending on what's happening, sometimes the TV will be on for several hours. Some days, it's only on for Play School. I dunno, I think everything in moderation. But that's just my opinion.

So to answer your question, I can understand why you may be shocked but I'm not sure it's your place to comment on her parenting approach.