> A former employee can sue for being fired or harassed
Yes. This is where all the rules come from. It's just like airline safety or workplace safety: for almost every rule, you can find the incident which caused it to be put in place.
The problem here is you not believing sexual harassment is real, or a real problem. There have been enough women bullied out of jobs they loved by harassment from male co-workers, over and over again, and enough of them have won their lawsuits or tribunals against the employers, that there are rules against this, which will be enforced.
And enough people will have used "I was drinking" as an excuse for harassment that companies feel the need to limit their risk by banning drinking.
People get annoyed at HR for being a "compliance" role, but it's the same phenomenon as hating the people checking the fire exits, the ladders, PAT testing all your electrical devices (this may be UK specific?), making you wear a hard hat and safety boots, or making you reconcile the cash in the register with the receipts.