When you consider that LHO apparently kept some of the photos of the Walker residence. And, if I remember correctly, that Marina reportedly said that she hid the letter. It appears to me that LHO didn’t necessarily “allow” it; rather it was her will possibly against his nonchalant attitude. And it appears that she won.
Marina's explanation about the note:
Mr. RANKIN. Did he ask you to return the note to him?
Mrs. OSWALD. He forgot about it. But apparently after that he thought that what he had written in his book might be proof against him, and he destroyed it.
Mr. RANKIN. That is this book that you have just referred to in which he had the Walker house picture?
Mrs. OSWALD. There was a notebook, yes, that is the one.
Mr. RANKIN. What did you do with the note that he had left for you after you talked about it and said you were going to keep it?
Mrs. OSWALD. I had it among my things in a cookbook. But I have two--I don't remember in which.