I'm going to address your last sentence first. What a person might think does not necessarily equate to the truth of a situation. A person can have little or no knowledge of the principle of homesteading, which seems to be the case with Bundy, but whether the principle is true or not has nothing to do with this person's knowledge. If they homestead unowned land, but have no knowledge of the homesteading principle, does that mean they did not homestead the land? Now you can disagree with the principle, of course, but you cannot deny that, whether they knew this is what they were doing or not, the ranchers clearly homesteaded land that was unowned and unoccupied at the time.
Now to address your receipt argument. This goes back to the principle of homesteading, and whether or not you agree with it. In my opinion, the people who mixed their labor with this land originally are the proper owners of the land regardless of whether they were given a "receipt," or some other form of arbitrary sanction by the government. There is clearly nobody with a better claim, at the very least. The simple fact is that there is no "receipt" because they are the original owners of the land. I only received a receipt to prove my ownership of this computer because I am not the original owner of the computer, but they were the original owners of the land so there was nobody in any position to give them a receipt.
Now it would at least appear as if you believe that any land that is unowned or unoccupied is somehow automatically the property of the federal government, but I don't see any logic to that position. You can correct me on that if I'm wrong.