Seems to me the memo line is like a note to self to remind the check writer what the check was for. It has no bearing on the receiver of the check.
After 9/11 and the Tsunami, the Red Cross was all over the radio telling people not to write specific disasters in the Memo field of the check. If you were to write Tsunami Relief 2004 or something of that sort, the ARC is forced to use the money for that cause. If everyone did it, they wouldn't have any money to keep operations going.
As explained to me by my friend, a business and finance major, when you fill out the memo field of a check, that is in essence a mini contract that you are entering into with the person who cashes it. By him cashing it, it's as if he agrees that is what the money will be used for. (Come to think of it, I should ask my lawyer brother instead of my business friend).