I won’t name the pub, but it’s one that was “knocked through” about thirty years ago, so is basically all one bar, but with distinct “lounge” and “vault” ends. At the time, it seemed a bit drastic, but has become more acceptable and familiar over time and now is certainly far more pubby than the typical “dining pub” makeover.
The sign is suggesting that dining customers entering from the car park – i.e. most of them – should avoid the back entrance but instead go round the front to avoid having to go through the public side and possibly encounter some slightly rough people. Not that it’s at all rough anyway.
While I can sort of understand the motivation behind it, the idea that it is likely to work on any kind of level except creating needless annoyance is incomprehensible.