GerMan - yes a lot of people do genuinely like unaccompanied song. And some people just can't abide it. Its personal choice. Probably coloured by the quality of the performances they've heard.
Unaccompanied singing is very naked - and there is nothing to pull the singer back into key if they should wander off. A bad unaccompanied singer is more unpleasant/difficult to listen to than a bad accompanied singer, because there is nothing to distract you from their faults. A good unaccompanied singer can be stunning - because, as Nigel says, the singing is not obscured by accompaniment.
For some reason it seems more impolite to ignore an unaccompanied singer than an accompanied one - a guitar & voice can more readily be consigned to the background.
As to ignoring a guitar - dispair not! They may be half-listening and loving it.