Quoted:
I have a couple questions, they may be silly but here it goes:

1. Are T-Marks USGI / Mil-spec ?

Some are and some aren't.

2. Why would a company not have T-Marks (laser engraved or machined) on an A3 Flat Top receiver ?

Cheaper.

I'm not necessarily on the Mil-spec bandwagon, but I do like to invest in quality parts.

Thinking about my second question, only thing I can think of is increasing profit of the receiver by skipping an option.