If one is truly working in the best interests of clients then there is no reason to fear being held to such a standard – unless you have some conflict of interest you’d rather not see the light of day.
It would be one thing if you knew which was which, it’s another issue entirely when this major difference is obfuscated – the major difference of course being whether they must work in your best interest or not!
The reality is that the advice clients would be getting under a fiduciary rule would be cheaper, free of conflicts and tailored for their benefit, not that of the salesperson.
As a plan fiduciary you are required to act in the best interest of the plan's participants. Failure to maintain a sufficient bond can be considered a breach of fiduciary duty.