I don't know why people are against this, it seems reasonable to me. If anyone goes beyond the intended usage (through themes or other modifications) then the developer shouldn't be resposible for supporting it
Theming ("branding", as they refer to) is, in a sense, just the core of their marketing campaign.
This is not just ideology - they do try also in practical ways to prevent third party devs from customization.
For a background of how horrible the people involved are, you can start from https://igurublog.wordpress.com/2012/11/05/gnome-et-al-rotti....
Here's the antisocial answer of one of the GNOME devs:
>> mccann replies:
> I guess you have to decide if you are a GNOME app, an Ubuntu app, or an XFCE app unfortunately. I’m sorry that this is the case but it wasn’t GNOME’s fault that Ubuntu has started this fork. And I have no idea what XFCE is or does sorry.