This is a somewhat sensible reaction, on numerous levels (though, ironically, not as much with respect to security). First, there is a small increase in security, but it also struck me as being more effective as a piece of security theater. In the NY Times article reference above, a police officer carrying a submachine gun is pictured. One of Kaedrin's 3 loyal readers wondered if that was really necessary. The truth is that it probably didn't provide much in the way of extra security, but often security decisions are made by those who have an agenda that encompasses more than just security. In Bruce Schneier's excellent book Beyond Fear, he calls this sort of thing security theater.
In 1970, there was no airline security in the U.S.: no metal detectors, no X-ray machines, and no ID checks. After a hijacking in 1972 ... airlines were required to post armed guards in passenger boarding areas. This countermeasure was less to decrease the risk of hijacking than to decrease the anxiety of passengers. After 9/11, the U.S. government posted armed National Guard troops at airport checkpoints primarily for the same reason (but were smart enough not to give them bullets). Of course airlines would prefer it if all their flights were perfectly safe, but actual hijackings and bombings are rare events whereas corporate earnings statements come out every quarter. For an airline, for the economy, and for the country, judicious use of security theater calmed fears... and that was a good thing.I wonder if the submachine gun the police officer was carrying was loaded? I would assume it actually wasn't, as a submachine gun is about the worst thing you could use on a crowded mass transit system.
The important thing to note here is that security decisions are often based on more than just security considerations. As security theater, Thursday's heightened alert level reduced public anxiety. On a more cynical level, it's also an example of politicians and businesses hedging their bets (if an attack did come, they could at least claim they weren't caught completely off-guard). Sometimes, those in power have to do something quickly to address a security problem. Most people are comforted by action, even if their security isn't improved very much as a result. However, as Schneier notes, security theater is largely a palliative measure. In a world where security risks are difficult to judge, security theater can easily be confused with the real thing. It's important to understand such actions for what they are. At the same time, it should also be noted that such actions do provide some value, often extending beyond the realm of security (which can be important too).
Update: Minor additions and grammar changes.
Update 7.22.05: John Robb notes the added cost (i.e. the monetary cost, the inconvenience, the civil liberties etc...)of the extra security measures implemented as a result of the recent attempts in London, and how the costs have spread throughout the US. Robb also notes that Schneier himself has commented on the specific measure of searching bags. To clarify my comments above, I think the value provided by Security Theater is, at best, a short term value, depending on your perspective. Is that value worth the added costs? If you're a leader or politician, probably. If you're a commuter, probably not. Politicians and other leaders usually have a different agenda than commuters, and they're the ones making the decisions.