SIMs are needed
I have been living here for nearly 4 years now and 2 of them have been spent using my own transport; a motorbike.
Buying a bike is easy. Everyone will sell you one in cash. Credit is not so easy. Nor is ownership of a bike. You can buy one but getting it in your name is hard and expensive and whilst can be done, it is easier if you have a trusted person have it registered to them.
If you own a vehicle you will have a STNK card which shows you have the right to ride the bike or car etc and this needs to be carried on you at all times when travelling. The police do do random stops and check for suck things. If you are western then if you are stopped, you will be either arrested and asked to pay cash, asked to pay cash and leave or asked to go to a police station and pay. In all ways you pay. If you dont have an STNK card then you might have the vehicle confiscated and be arrested for theft.
If you get stopped you also need to prove you are allowed to drive. You need a licence. I dont have an international licence and so cant comment on it, but I would say that the police will not understand it. So better you try to get a SIM or licence from a licencing center. I got mine in West Jakarta and went through an agent for the first time. A SIM normally lasts 5 years, but because we are western 1 year.
Going with an agent will cost you about 600,000 rupiah and takes about 2 hours as they rush you through the photo graphs and all the paperwork is done for you. You need a copy of your passport and Kitas and not much else. Going without an agent but using the police contact will cost about 400,000k.
I cant say the process is legal however it does get you a licence to ride bikes or drive cars, which ever one you need and the police cant do anything about it. You can also book airflights with it as it is a legal id card.
The licence is also very important if you are planning to ride or drive in Lombok or Bali as the police have in the past been known to stop and fine motorists for not having the right paperwork, even though the man that hired it to you did not ask.
So now you have all the legal bits of paper to ride and drive in Indonesia, good luck. Just slow down and dont take risks. If you dont understand just sit and watch the traffic move for a while, then you will know....
