I know that the combined cost of landing fee and museum entry is expensive for some (even though you've probably just paid several times that amount for avgas) ... but that money goes to keeping this world class museum running, so you can keep enjoying it. We can't expect it to have such high quality exhibits for long if everyone tries to get in for nothing.
BTW, I'd heard about the free entry with a landing fee, but I'd heard one hour. Perhaps alphaalpha can tell us.