They weren't issued ammunition.
It would have been dangerous and pointless to do so.
This is an example of "security theater," the use of an apparent, but ineffective security measure. It's an example of good security theater, because it reassured people without endangering or inconveniencing them.
Aside from securing the cockpit doors, all the air travel security nonsense we're subjected to is bad security theater. It's inconvenient, expensive and creates petty authoritarians, while doing nothing to make us safer from a very low probability threat.
In fact, much of it seems to be perverse, to make us feel less secure, more conscious of a very minor threat than we need to be, to enable an authoritarianism that is decidedly not petty.
Late update: Last week, I put up a post linking to an article by Bruce Schneier (most recent blog post serendipitously relevant) about border searches of laptop hard drives. Commenters expressed doubt about one part of the article, that British border security was looking for porn.
I sent Bruce a note asking for backup. He, eventually, sent a link to his NYT source, which included:
'The question of textual pornographic material is more difficult. "By and large you can probably get away with a lot more with text than you can with pictures," Thompson acknowledged. "But if we thought that any material was obscene, we would seek a legal opinion on it if it was a borderline case."'
There is an argument to be made that consumers of child pornography should, like consumers of elephant ivory, be prosecuted because the production process is repugnant.
Text? WTF?
Update update:
Bruce wants you to read this one.
--Major Major