This sentence is not clear so let's go by the wiki...
I cannot think of any other support job that cares about the full (main) level of the job is. Every support only job is leveled to 37. Why should /BST be any different? Why do I need to spend another 50 hours (500k exp) just to maintain my current playstyle?When Beastmaster is set as a support job; Charm is affected by the main level of your Beastmaster job.
For example, if your Beastmaster job is level 40 and is set as a sub for a level 75 job, a monster that checks as Easy Prey will probably be very difficult to Charm. However, if your Beastmaster job is level 75, and if you set it as a support job to another level 75 job, an Easy Prey may be Charmed with ease.
This was a case of bad design. The server philosophy allows identifying bad design and balancing accordingly.
I suppose a counter argument for this was so that a high level character could not charm anything EP+ as 75JOB/BST1. The success should be proportional of the levels towards capping BST as a support job. So you could still penalize an underleveled sub job BST on their charm level check with this formula:
effectiveCharmLevel = mainJobLevel * subJobLevel / subJobCap