I've been offered a magazine subscription at a discount.

Tux magazine by any chance?

Very nice and fully intended on taking it up. Until I went to the sub's page
and found that the offer was only valid for potential US customers :-(

Interestingly I've come across this annoyance a few times myself over the years. I don't think there's any sort of legal requirement to inform non-US residents that the offer only applies to the USA however it does strike me as poor marketing skills.

No word of this in the initial e-mail and in none of the other three
from a series of months ago... So they've had time to amend the way
it's written. Ok, it's the US but I understood that they had to
comply with our advertising rules too?

There has been masses of debates about legislating the Net, it'll never happen, or at least not in the way your thinking the world is still, unfortunately, of the nationalist mind set when it comes right down to it, rather than a global one so whats not allowed in one country is allowed in another and seeing as no one person or country owns the net enforcing rules is pretty much impossible other than blocking them like China was doing with Google until Google agreed to censure some of its content for the Chinese market.

Even some of the awful porn websites cannot be shutdown so all our efforts go into catching punters who visit and view the content instead. The web is brilliant because its so difficult to control. You cant ring up Brussels and say "Oi that rotten man is telling fibs about membership on his site" they'll just say "Sorry you'll have to contact the sites webmaster or ISP" until of course Microsoft reveals that in fact it owns the patent and licenses on www. :-)