Hmmm... its a different question now! Will choose to post a new response rather than updating the earlier one :)
Not very sure of why MS took that approach. Could be because implementing it on certain fields might break the OOB CRM functionality.
Well, there are different approaches that people take to implement FLS for OOB fields. If you are okay with unsupported customizations then here is a simple way of doing it. http://weblogs.asp.net/gayanperera/archive/2010/11/02/crm-2011-field-level-security-for-oob-attributes.aspx
This is being tracked as a suggestion to MS on connect. You could vote for it to get into the product in the upcoming versions
https://connect.microsoft.com/dynamicssuggestions/feedback/details/650426/field-level-security-on-system-fields
Hope this helps!
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