Yes, it does pay to turn off your brain and just take these movies for the schlock they are. Exhibit 1: Mankind can find a way to round up and control the beasts, and corral them on Monster Island, but once the monsters are loose humans can't find a way to dispatch them.
Because the aliens were controlling the beasts, so mankind cannot regain control of the monsters. It is only until they find the aliens' source of manipulation that they were able to regain control of the monsters.
Yes, but the security devices on Monster Island make it clear the monsters were not there (and staying penned up) of their own volition to start with, so why would alien mind control over the monsters matter? (damn you, brain, shut the hell up)
The monsters were not on Monsterland in the beginning because the aliens manipulated them and made them escape; the monsters didn't leave the island on their own free will. The manipulation started when the aliens unleashed the gases throughout the island, controlling both the monsters and the humans inside the control center.
Like OllieSuave said, the humans couldn't control the monsters when they were outside the island because the Kilaaks were manipulating them. The island's force-fields and gas-traps were made obviously to only to keep the monsters inside the island. It was assumed that the monsters would not be able to escape since the control system was working smoothly; they would not have predicted that aliens would attack. However, the humans did make a seperate system to control the monsters from outside the island. They were only able to use it when the Kilaak's control operator over the monsters was destroyed, which enabled the humans to regain control of the monsters. They did have a system to control the monsters outside of Monsterland; they used it when they pitted the monsters against King Ghidorah.
I've got to agree, controlling the monsters aside (aliens, humans, whoever), it seems pretty silly that they still couldn't use the gas traps and what not to repel the monsters from Tokyo or wherever. But whatever, it's not too important to just enjoying the film. Though I don't like it as much in films like this or Monster Zero where Godzilla is portrayed as a brainless puppet who can be easily controlled or manipulated. I much prefer the portrayal in films such as Godzilla vs Biollante or Godzilla Final Wars, as a powerful, unstoppable, unpredictable force that cannot simply be contained or controlled.
Well you could say that they could keep the monsters at bay in the island because they were acting under their own instinct. If you prod something with a stick it may think, won't bother going that way, but if it's under mind control it will carry on going no matter the pain or annoyance.
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Even if you hate Uwe Boll, give Postal a try, be offended or entertained.
Actually, humanity is unable to think or do anything of much interest because their minds are obliterated by superior alien intelligence. Therefore the Monsters have the run of the place because they're smarter than the aliens, who are dumb enough to fool with humanity in the first place.