And put those incredibly easy to open windows on the habitats?
It's because the film takes place in a world full of morons. There, your question answered!
An emergency exit, perhaps? All attempts to open the main entrance failed, so they would need a way out if something went wrong.
Being such a large project involving hundreds(?) of people, this could probably be chalked up to a few careless construction workers and inspectors, especially since said workers may be thinking of this as a fairly superfluous project.
What was their excuse for keeping them in the dome, again? Oh yeah, that's right, letting them out would mess with their data. MORE THAN HAVING TWO COMPLETE IDIOTS RUNNING AROUND AND DESTROYING EVERYTHING WOULD.
They should have ended the project immediately after the "heroes" attempt to rape the two female scientists. At that point, the intruders are known to be dangerous criminals who are deliberately seeking to harm those within the dome.
Not even the actual biodome project this film was based on was that asinine about not messing up their data. A few weeks into the project and a guy was badly injured and needed to be taken to a hospital. But him leaving the dome didn't end the project, they just went on as normal without him.