If you consider things carefully, you will quickly realize that ASICs do not make the network any more secure. Yes, it's quite true that they do allow the "good guys" to have a much higher hash rate by purchasing them. On the surface, this probably seems like it makes the network more secure. But there is a problem. The "bad guys" are able to purchase the same ASICs as the "good guys". They would be attacking with weapons of the same strength, security is not affected by whether we use ASICs, CPUs, or GPU's. Everybody has the ability to purchase the same devices, therefore the relative numbers don't really change, only the absolutes.

Let me give you a small analogy. You'll just formed a small country, and you have to defend it from intruders. No weapons have been invented yet, all you have to fight with is your fists, this is also the only weapon your enemy has. Okay, so now clubs get invented, you get all your people armed with clubs now so that your nation will be more secure. Here comes your enemy to invade, seems they've got some nice clubs too. Well, it seems like your security is not much better than it was before, doesn't it. Well, you just heard some encouraging news. Somebody is just invented a much more powerful weapon called a gun, get a few of these and those club wielding invaders won't have a chance. So you get yourself armed with these new guns, you feel very safe. Then the enemy invader show up again, they don't seem to have any clubs with them, what do you suppose they are carrying?

You understand the point of this little story? As long as the same technology is available to all sides, nobody gets an advantage. This is exactly how ASICs "improve" network security.