Hi again,
I am sorry to rock your boat, and with all due respect for your professional knowledge about immigration (I found a factual mistake on your web site, by the way after checking it out for only a minute and than didn't bother anymore, sorry), but if you check newcomer link: http://www.immigrationboards.com [nonactive] you'll see that people on the HSMP visa have been working as cashiers at TESCO for a couple of months (for example) during their first 12 months (and nowhere else), got their visa extension after 12 months, and shared information about that at newcomer link: http://www.immigrationboards.com [nonactive] . There are also some other immigration experts advising on that board explaining how salary and job level (and even if HSMP holder worked or not) during first 12 months are not controversial when it comes to extending the visa, as long as a visa holder did not use public funds, was looking for work, or was economically active (in any job/position not just "your field").
Also, I have never said in what particular field I am going to work when I arrived to the UK as an HSMP visa holder. Having two degrees (in different fields and from different countries), an MBA and some other diplomas (one post. grad) in between, I really can work in at least 5 fields to be consistent with my education and experience, which was backed up with my documents in the HSMP application process. So that "in your field clause" you mentioned is very transparent in many people's cases and (what is more important) is not mentioned anywhere in the official Home Office documents related to the HSMP. If it is, please share them here with us.
To go back to the Chinese gentleman with PhD and HSMP visa... After looking for work and not getting it in your field(s), one must feed a family and can end up as a cashier at TESCO for example. Many of the HSMP holders did exactly that but managed to get their HSMP visa extension. Nobody's crazy (like a doctor from your example) to work as a security guard just for fun while being of some respectful profession at the same time. Of course that everybody on HSMP visa is looking for work in their field, but if that doesn't come as planned, the UK government will still issue an extension after 12 months if you prove you've been looking for work "in your field(s)" while working as a cashier at TESCO. Even if you didn't work at all (but maybe had enough savings to live on them during 12 months), but made an effort to find a job, you will still get an extension. Under "made an effort" the government needs to see where did you send resumes, if you had interviews, registered with recruiters etc. So, the Chinese applicant with a PhD and an HSMP visa can work anywhere and for any salary (which was original question posted here), and still get his HSMP visa extended (which is of only that guy's concern and not of his employer's). I doubt though that he is also not looking for work in "his field" at the same time, which is a quite normal thing and would be irrational to think he's not.
The bottom line of all of this - what is the reason that an HSMP can work in any position (or not at all) and still get an extension? The Home Office knows that very often 12 months is not enough for people to get settled in HSMP holders' original "fields" of work (sometimes due to ignorance that UK employers show when stumble upon an HSMP visa holder as they are reluctant to hire those applicants when they see that initial visa is for only 12 months and are not sure if the applicant will still be in the UK after that time), but as long as the HSMP holders did not use public funds (even if they work they are not entitled to them!), their visa extension will be granted as the UK needs highly skilled people for the years to come. So, maybe that HSMP holder couldn't get a job during those 12 months, but he/she will one day... In my personal opinion, the fact that visa is stamped for only 12 months initially is a huge obstacle for most HSMP holders as they need to be lucky to find an employer who will understand that it can be extended after that easily and are willing to give them a job in "their field".
In any case, this situation in the UK is still much better than in Canada where highly skilled professionals trained/educated in other countries and without that infamous "Canadian experience" have minimal chances to work in their fields at all. Canada has too many doctors/engineers/architects/teachers... driving cabs and delivering pizzas or working at factories for minimum wage that it is scary, but that is another story...