The problem is that Elly is not rational. As I've said before, she finds the high speeds and fear of injury I've mentioned so terrifying, her already tenuous grip on reality slips and she turns into an even more raging and panicky asshole than she normally is. As an example of this need to not consider the rational side of things, let us remember something that Elly tends to forget. This known known that Elly doesn't want to know is that even in the shiny metal box of being protected, one is not invincible. While she was screaming at Liz about how she wanted to die and make her poor mother sad, she conveniently failed to learn the lesson she should have learned from Deanna's damned near dying despite being in an inherently safe car of never coming to harm.
This, I should think, is because Elly seemed to see Liz's wanting to ride the motorcycle as rejecting the protective goodthink coming from her loving mother's great big heart. As long as her little girl was in the protective car of being surrounded by Elly's love, she was safe. What happened to Deanna was obvious to Elly: Mira simply didn't love her enough to send her protective thoughts that would protect her from the laws of evil, conflict-causing man physics.