The truth is, no matter where they make the security perimeter, there is always a chance for attack of the masses. However, there are many such places in the US like this. New York train system/grand central for one example.
The hope is to to discourage the ease of it. Anyone truly determine to do such a thing, could likely find a way. A shooting at disney, whether it involves 10 people or 1000 would be big news. The large amount of children likely involved would make for a very politically charged reaction, if the attacker was backed by a foreign entity.
Like any place that has security screenings, it an odds thing. All the players know it will never be 100% effective. The hope is that a potential bad guy calculates the odds of getting through security and opts for an easier target elsewhere.
My opinion is that it is mostly a spacial issue. There is back up as it's done currently, making for an unseemly appearance at the front gate. The crowding may also be unsafe in general, but it certainly is not guest service friendly.
If you can move the screening to a place people are already waiting for something, you'll make a more efficient process. Basically if you have to wait 10 minutes to board a ride to the park, you might as well kill the time getting screened, rather than wait 10 minutes to board, then get to the park, then wait 8 minutes to be screened.
If there is one absolute at Disney, they'd rather have you spending your money in a gift shop, or restaurant or anywhere. If you are in line you can't spend money. There is a cost benefit for them to do this. If it gives you the perception you are safer, then it's a side benefit. Make no mistake, someone has analyzed the financial implications, and they've acted in a way that helps the bottom line. Sometimes what is good for disney's wallet is also good for everyone else.