The amount of FG a user has is public for several reasons.
1 of the many reasons is found here in the Rules:
https://forums.d2jsp.org/info.php?p=6Quote
Do not offer fg/items you don't have.
2nd of many reasons is to prevent scams (although all Trades are recommended be done via a Mediator) but the display of FG can help prevent Scams. This helps other users "judge" another users "experience & trustworthiness".
Displaying FG is not the same as displaying your bank account as your FG logs are NOT publicly displayed.