Liquid fuels always have a danger of ignition. You just have to have enough heat to get them started so you should always handle them with care. But that's not the immediate issue here.

The MSR fuel bottles are available from REI also. They are an aluminum (thicker wall than a pop can) bottle with a nice tight cap so that they don't leak if closed properly. Things get knocked around on a bike. Stuff rubs against other stuff and the next thing you know, whatever is riding with the fuel is covered with the fuel - if you use plastic bottles! And you are eating cold dry freeze-dried for dinner! That's just icky!

The metal bottle will just stand up to abuse much better than plastic. The thin plastic bottle that isopropyl alcohol comes in is packaged in a box so that it doesn't have to rub against other stuff in transit. It wouldn't last too many miles in a bike bag.

It's just better to be safe than eating dry freeze-dried food.