@Paul *******
I have a theory about visa exempt border runs. If you are going to end up being tax resident or look like you're at risk of it then you run a good chance of being denied entry. For example if it's June and you've already done over 100 days in the country then at the rate you're going you are likely to end up tax resident. (180 days in a calendar year) So I think it's rather the case that they don't want you tax resident on visa exempt entries for one reason or another.