@Mike Walker
Is this tax a one off, can you re-enter Mexico again with the same housing and not pay tax a second time?
Not that I'm likely to travel there but just curious.
Speaking for Canada (and I don't think this is unusual elsewhere): Typically once you've imported a good and paid duties and taxes you do not have to pay those duties/taxes if it enters again, for example, when you send it off to the manufacturer for repair or calibration. I'm far from a customs expert, but I do this occasionally for business. You are, however, typically taxed on any 'improvements' (billable repairs, calibrations).
That's how it is supposed to work.
I think the underlying takeaway in my post above is that, at SJD, they will do whatever they want. They are not following their own laws and you, as a recreational traveler, have no viable means of contesting the decisions made on-the-ground in the customs hall.
The more people that waste their time arguing with them, the more people that make it known they are not travelling to Mexico because of these policies and the more people who request chargebacks (and hopefully win) the better the odds we will see some sanity. For now, however, it seems like one must accept it as part of the cost of the trip.
I don't think this is likely to change. We are too small a demographic to matter. If anything, it might expand to other ports of entry.
Hopefully this conversation continues and someone eventually finds another solution or line of reasoning that they accept.
There are less convenient but alternate ports of entry in the area if you are flexible in your schedule. Or routing through MEX.
Stories of 'I got lucky and they didn't check' or 'I lied to them' or 'I bribed the guy' are all suffixed with 'that one time' and just perpetuate the underlying problem.
Perhaps the operators down there can work further up the bureaucratic chain and get some real answers and change.