Potjeh wrote:But making crossroads walk-through and placing travellers on their tile seems like such a simple fix

On first glance, yes. In practice, it has the following consequences: You'd have to make crossroads authority objects, to prevent easy griefing. This has the unwanted effect that crossroads can't be easily destroyed by their owners anymore, to prevent security vulnerabilities.
On a more practical note, currently only HF are walkthrough buildings, I believe? Since they are a special condition, even in that they are only walkthrough for the owner, it's probably bluntly hardcoded somewhere*. If loftar were to add more walkthrough buildings, he would probably want to do it properly, and integrate it neatly into the rest of the code. This means thinking about data structuring, and how to record the distinction between walktrhough-for-everyone and walkthrough-for-owners, and would be wholly a lot more problems than it seems on first glance.
This could all be ignored, and it could be implemented as a quick 'fix'. But with large projects like this, using quick fixes do more harm than good in the long run.
*No offense, but in my experience, that's just how things go.