can offshore worker under T1G apply ILR

[Jasmine2013]

Hi, everyone, I am confused about one sentence in the guidence on calculating continienty:
"Any time spent working off shore on the UK continental shelf, beyond the 12 mile zone defined as UK territorial waters does not count toward the continuous qualifying period for ILR, for example on ships or oil rigs. You must count this as an absence from the UK."
This is very odd stipulation, if one is working offshore, he pays tax like the others, but cannot apply to ILR under this rule?

So if a person is under T 1 genderal visa, wokrs as offshore worker, he is not eligible to ILR?

But, there is no records of going to offshore in passports, like stamps when we go abraod. In this case, how can the caseworkers know one is offshore worker or not?