This has always been an issue as there seem to be some who think it is okay to instruct on a milonga floor, rather than reserving it for a practica or a private lesson. Recently I have seen this happen openly and covertly. When it is done openly it can disrupt the dance floor - and upset the vibe as well as the other dancers. When it is done 'undercover' as it were, it can also disrupt things - as I also saw recently where a dancer was obviously obliged to dance with the one person (I presume they were being paid) and was instructing their partner on the floor.
Practising moves off the dance floor - in a corner of the room for example is fine, but practising and/or instructing on the floor is not good manners. I am surprised that some think it is okay - just as I am surprised to hear one teacher touting for business at another teacher's milonga! I accept that tango is a legitimate business for many teachers - and organisers - but I don't accept that it is okay to use other's events in this way.