Hi Jonathan, the reactions you point out are typical. Until they see it with their own eyes, few believe that the Indonesian authorities can/will go after resident expats for taxes owed on their international income.
However... this year the Common Reporting Standard has come into effect. 47 countries have signed so far, including all OECD countries. Indonesia is a signatory. Under the CRS, countries will share individual financial information with each other to allow all to better enforce their tax laws. So, if an Indonesia resident expat from Australia, for example, earns income in Australia from a bank account, business or investment, it will be reported to Indonesia (along with the balances), so that Indonesia will then have the information necessary to assess income tax and, if relevant, penalties. This will likely occur first with Kitas and Kitap holders i.e., those with long term residence permits, and it is not inconceivable that Kitas and Kitap holders will eventually have to register for a tax number and certify tax compliance as part of the application and renewal process. As to the others who 'live' in Bali on the visa run method, it will be more difficult to enforce. One option for the authorities would be to limit the use of these visas so that an expat could not legally stay in Indonesia under them for more than 183 days in any twelve month period. Given how much these types of visas are currently abused, I would not be surprised if this happened.
At the moment the CRS is the only realistic way for Indonesia to enforce its tax laws against residents having overseas income. Other alternatives would probably require more manpower and technology than the tax office currently has. Having said that, the tax authorities recently warned Indonesians that it is obtaining AI software to monitor social media accounts to see whether an individual's lifestyle and network of relationships is inconsistent with his / her level of reported income and assets.
http://www.thejakartapost.com/news/2018/08/30/tax-office-to-spy-on-taxpayers-social-media-activity.html
Bottom line is that the government is highly motivated to improve tax collection, and so the net is closing, slowly but surely.