Answer: Being in the left portion of the lane kinda encourages that. If you were driving a car and there was space to your right for a right turning car you'd see that as normal. In fact, that's a feature of being in the left potion of the lane, it allows for drivers behind you to do right turns on red.
What's not normal is if a driver comes alongside your right and doesn't turn on red. They could choose to go straight and pass on your right. When the light turns green be hyper vigilant so you don't move to the right and end up in their path after they've already started to overtake you. If you are being overtaken on the right go slower so the danger passes quickly.
To prohibit right on red you'd need to be on the right side of the lane but the trade off for that is increased likelihood of a driver coming alongside your left and passing improperly in the intersection or worse turning right, across your path. That's known as a right hook.
A hard part of driving a narrow vehicle is lane position is a bit of an art.