If you test negative coming in to the island I don't understand the reason for (D) above. Am I missing something?
To account for false negative PCR’s and turning positive and/or becoming infected after testing... The current regulations with a semi-quarantine for 7 days and retesting are meant to strike a "reasonable balance" as detailed below.
A pre-trip PCR screen keeps some infected individuals from even traveling but the limitations of PCR testing are well-known. Under the regulations of 1/19/20, the PCR serves this purpose but is not considered sufficient for the traveler to go to restaurants, bars, etc..
Studies indicate (confirm the intuitive) that a post arrival quarantine is better than PCR screening.
After a quarantine, one essentially becomes part of the population and is treated as such.
The European CDC has issued
a fairly comprehensive guideline document that includes discussion of strategies (most pertinent part starts on page 7 and includes source references). A short excerpt (underline added):
"Modelling studues... have explored different aspects of the effectiveness of various combination strategies... Based on these studies, a 14-day quarantine period appears to be most effective in reducing the risk of transmission from travellers, although this creates logistical and financial challenges. A 10-day quarantine (without testing at day 10) seems to be the next most effective alternative, particularly if used in combination with other non-pharmaceutical interventions or where countries do not have enough testing capacity.
Alternatives involving shortened quarantine or combinations of quarantine and testing have been also explored, and it is expected that these will have less negative social impact. If a Member State implements quarantine of travellers, testing could help shorten the quarantine period. A single test upon arrival is considered to be about 40 to 50% effective in preventing the importation of the virus. Given the likely low prevalence of infection among travellers, and assuming that contact information is collected for travellers to enable follow-up if required,
the combination of quarantine and a single test at around day 7 after arrival appears to offer a reasonable balance of risks and benefits as an alternative to longer quarantine without testing... Testing twice (upon arrival and after a few days to release from quarantine) does not seem to significantly increase effectiveness - compared to testing only once to release from quarantine - and it is logistically challenging and more resource intensive."