well rogue, they don't write about "serious issues of professional misconduct", as you put it, but rather about "serious issues of professional conduct". kind of implies, together with the rest of the text you posted, that there are serious issues but it is not NECESSARILY and automatically "misconduct".
the excerpts you posted very clearly advise against it. but they do not state that it is to be avoided in all circumstances. they give guidelines about what to do if it happens, implying that it can occasionally happen, rather than just saying "it's not allowed".
further, in relation to an earlier question. in Switzerland and Germany MOST PhD students have the status of staff. i personally find it extremely weird and experience it as a blast to my confidence that over here, what i do does not get the status of "work" at all. i concede of course that i am learning. but so are most "staff" in most jobs.

don't get me wrong, i'm not suggesting that lamp should pursue this thing. there are other issues here, too, and it would definitely make things complicated. but your TOTAL opposition to staff-phd student relationships strikes me as overly "politically correct", seems very "american". like apparently profs won't close their office door when they have a discussion with a single student (a typical american clichee). in europe, continental europe particularly, such behaviour simply appears as odd. not saying one is better than the other. just saying that not the same values are pursued everywhere, not the same things are seen quite as equally problematic everywhere.

They don't say outright that it's not allowed because it is a sensitive issue, and not because they 'may' allow such things. And whether you consider yourself staff or not, if you read the excerpts they refer to, and described staff-staff relationships as well.

I'm not against staff-staff or student-staff relationships but it IS serious professional conduct in the way that katq described it where people doing PhDs are openly allowed to be in a relationship with their supervisors, even going so far as to say it made 'interesting gossip' which sounds like she's completely oblivious to the seriousness of her claims.

This isn't about cultural difference or political incorrectness or sexual harrassment as you described in your 'american' scenario. This is about professional conduct, the standard of which is the same no matter where you are.