The ROI of banning VPNs is very minor...What would they get in return? Providing investigators with easier access to information they can already get? Whereas banning VPNs would be an expensive legal and enforcement hurdle and has the potential to cause media/civilian backlash.
So why ban VPNs when they are already expanding they're existing ability to:
a) get access to information held by service providers (which include VPNs)
b) "remote access to computers to downloading covertly the contents of a mobile phone during a search."
In practice, a VPN does nothing to prevent them from accessing the sites you visit. Other than requiring some additional paperwork to send an information request to VPN (not even a warrant) to fill in the gaps which passive surveillance can't provide. Plus the VPN will give people a false sense of security... so they wont think twice about visiting [verybadsite.com].