While getting inside the forest may sound easy, it is harder than it should be, namely because of the soldiers patrolling the area.
The reason the Betrum Nation told the others for the existence of the patrol groups is namely safety, as it would allow them to discover any disturbance in the forest.
However, it is a journalist secret at this point that the reason for the patrol is to make sure there will be no one sneaking in a getting their hands on the materials from the forest.
Betrum Nation has complete control over the market as the Forest of Bug is surrounded by steep ranges except for it's border with the nation.
It is this reason why Betrum Nation is the only one threaten by the Forest of Bug, and the reason it able to hold monopoly of the resources find inside it.
However, while the patrol could stop regular adventurers, against a demon, that bought the Tome of Darkness, he was able to slip by them and enter the forest.