A header is more than "one line", it is the major eye draw that makes someone take an interest in an advert sufficient to click through rather than discard it. This is why the position and visibility of a small mistake can be a major issue when it comes to the pretty strict rules we have here in the UK. If the line was buried in the email and people had to click through and ferret around to find it, it would be regarded as much less of a problem.
The other factor is that it's not an "obvious mistake", but a credible offer that is often made and honoured, and that players would believe was a genuine offer made in good faith.
If Tesco list a flat screen TV on their website for 4p, it's an "obvious mistake", and they wouldn't get into much trouble for refusing to honour it. (This has actually happened!!!). However, if they had mistakenly offered it at the price it was sold for in last months promotion, and a price they end up selling it for later on, it would not be an "obvious mistake", but an offer that customers were entitled to rely upon and take up in good faith (it might even be regarded as a bait & switch). Reneging on such an offer would be more troublesome, and in general, Tesco would eat the mistake and honour the deal rather than suffer bad PR that would only get worse when a couple of months later it was pointed out they were "repeating the error", but this time honouring the sales.
It would be better to avoid having too specific a message in a template that can be selected in error, but put the offer in the body of the email itself, where the content is far less likely to be a template.
The ASA would only say "don't do it again", they wouldn't issue sanctions for an isolated mistake. What the ASA are really trying to do is go after those businesses who have a policy of making repeated "innocent mistakes" in order to deliver bait & switch advertisements designed to get people to click through based on incorrect headline information when they would ignore the advert were the headline truthful.
We see quite a few casinos who repeatedly "use the wrong template by mistake" in order to drive traffic to a poor offer, and sometimes the switch only becomes evident once the player has deposited. The remit of the ASA was extended to cover the internet because of the number of businesses that would behave when advertising on TV or in printed media, but misbehaved in the wild west of the internet because it was unregulated and thus there would be no one to pull them up for it.
This is nothing compared to some of the major marketing blunders (as in "they got caught") made by the likes of, for example, the old Jackpot Factory many years ago when Americans could play and regulation was pretty lax. After all, 32Red haven't claimed that playing the new games will cure cancer or end all your financial troubles