Am I to understand that if one stays in the UK as a tourist for six months, leaves for six months and then returns, they will be subject to invasive scrutiny even though they adhered to the requirements?
The relevant wording in the advice the Home Office provides to those making the decision is
Are you satisfied they will not be living in the UK or making the UK their home through frequent and successive visits?
Which is decided on a number of points including
the purpose of the visit and intended length of stay stated
the number of visits made over the past 12 months, including the length of stay. on each occasion, the time elapsed since the last visit, and if this amounts to the individual spending more time in the UK than in their home country.
Someone who has spend half the last year in the UK is liable (though as noted in the comments not certain) to need to provide a clearer and more lucid explanation than someone visiting only occasionally. You should also note the statement that there isn't a formal rule such as six months on, six months off:
There is no specified maximum period which an individual can spend in the UK in any period such as ‘6 months in 12 months’. However, if it is clear from an individual’s travel history that they are making the UK their home you should refuse their application.