There are a few reasons engine braking is illegal in some places. One reason is that it can damage the engine. When you use the brakes to slow down, the engine slows down as well. This puts extra wear and tear on the engine and can cause problems over time. Additionally, engine braking can be dangerous if you're not used to it. If you're not paying attention, you could easily lose control of your vehicle. And finally, engine braking is simply unnecessary in most situations. There are plenty of other ways to slow down your vehicle without putting extra strain on the engine.