While you make some good points, wordbox, most people would never equate a short haircut with a tatoo necklace and a chest piercing. I wouldn't mind a bride's maid with short hair, but I would not like tatoos and piercings in my photos. I see nothing wrong with asking her to use a shawl or cover them with makeup for the photos. My sister had a bride's maid with tatoos, and they just posed her so that the tatoos were facing away from the camera. She didn't mind at all.
I don't think it's all that different, given the context of why it would upset the bride. Sure, it's just hair, but it's also just skin and jewelry. My sister was ready to try to control the the way I looked to fit her image of a bridesmaid.
It would have hurt me if she had tried to prevent me from getting my tattoo as well. I had a tattoo that showed in my dress, and while I would have been a little annoyed to cover it, I would have done it if she asked me to. But to face a certain way during pictures to hide me would have made me pretty sad.
To each their own, I suppose. Everyone has different ideas of what their wedding should be and what standards they will hold their wedding party to.