It's the use of the word OR that is the issue. It can be read that you can use the bag to kneel on OR to support the shin, either is OK. And that precludes any other use of the bag, either of those being acceptable.
There is a term in logic called EXCLUSIVE OR, which is what this phrase is being interpreted as, meaning one or the other but not both.
Example; I have a joint bank account with the wife, and the mandate says either party to sign cheques. Me, OR the wife. Now if we both happened to sign the same cheque, that would not make it invalid. If it had to be one signature only and the other appearing would negate the instruction, that would be an exclusive or.
I know which one I think makes sense.
edit: looks like Adam beat me to it