The syntactic category rule means that when two words are confused for one another the "target" (the word replaced) and the substituting word are almost always of the same syntactic category. In normal speak: nouns replace nouns, verbs replace verbs, and so on. If "Obama" were a verb instead of a noun (as in, the Democrats are going to Obama the GOP in 2012), we would be substantially less likely to confuse it with the noun "Osama."
The speaker is also subject to what linguists call "priming." Your brain makes certain words more accessible to your tongue when they resemble--in pronunciation, in meaning, in subject matter--words that you frequently hear. "Priming means that when you've been reading/hearing/thinking about hospitals, words like 'doctor' and 'nurse' will be recognized more quickly, and are also more likely to be substituted in a slip of the tongue," Liberman explains. So hearing Osama and Obama in the same context makes your brain more apt to use them interchangeably in speech.
Interesting. And, as I said on this podcast this week, it's somewhat understandable when average people make that mistake, but when professional broadcasters do it over and over, you have to wonder about either their motives or their professionalism.
If it's the former, they clearly intend to conflate the president with Bin Laden. If it's the latter, they really ought to come up with a way to stop doing it. For example, you're not allowed to say the seven deadly words on broadcast television and radio -- so you train yourself to not do it. Likewise, they should train themselves to not mistakenly discuss the "killing of Obama" or the presidency of "Osama."
For example, try calling the president "The President" or "President Obama." It's respectful and correct. And try calling "Osama" by the second two words in the westernized version of his name: "Bin Laden." Problem solved.
This is mandatory. Stop it.