Sometimes it is reasonable to allow certain things to be on outside walls. However, in this case, it sounds like a problem we have where I live. Various barnacles like gas water heaters and air-con compressors get put on the outside where everyone else has to look at them and not on the inside where the unit owner would have to look at them.
Typically this happens for several reasons.
1) The unit owner is selfish and prefers to have them out of sight except to everyone else. If it went inside, it would occupy some space that the owner would prefer to use for something else. It might require a slight rearrangement of the garden furniture, for example.
2) The unit owner might be a bit more sensitive but won’t stand up to an installer who says ‘We always put them on the outside wall.’ Usually that is because the installation is much easier and slightly cheaper, but it could have gone inside the unit area if the owner had stood up to the tradie.
3) The unit owner is acting in haste because they have no hot water and just do what the tradie says. Often the tradie will tell them that they don’t need permission. Of course the tradie wants to get on with the job so they are hardly motivated to explain the operation of the relevant strata act.
4) The unit owner looks around and sees another example of a barnacle on the outside and makes the self-serving assumption that the other installation must have been approved, or, if not, nobody has done anything about it so, obviously, you can get away with it.
5) It doesn’t occur to them that they need to ask permission from anyone and, if they do need to ask, then they find that an offensive affront to their views on civil liberties etc.