One has to pay Taxes on L1 & H1 visa
I have been on both H1B, L1A and B1. Unless the companies/individuals violate the law, one has to pay taxes either you are on H1B or L1A. Per Diem is paid only while your are on B1 visa.
However when you come on H1B or L1A visa, the company may pay an allowance (in addition to the taxable salary) as settling-in expenses (hotel stay, car rental and aprtment search expenses). Some companies may not base this on actuals (I think this is legal because IRS does not require reciepts for certain expenses that are under certain amount).
Only other per-diem I can think of is if an h1B or L1A is travelling on business to a location other than thier base location, you are paid for hotel, travel and food expenses. These are based on actuals excepting that some companies may not require employees to show reciepts for food (it is more of administrative work than it is worth). If this is illegal, then even green card holders & american citizens are also commiting the same fraud (IRS) H1B or L1a is.