Why my network work

This phone is almost certainly still carrier-locked to Boost; the U mark indicates it is a carrier SKU sold to the carrier to resell which ends up SIM-locked (for obvious reasons). 1U is the same as the U phones in this regard, so both are SIM-locked and sometimes used interchangeably as needed. The ONLY phones from Samsung without a SIM lock are the U1 phones which cannot be carrier-locked to anyone due to how the radio firmware works on these; they will refuse to lock to a carrier as the command is disabled at the radio level.

Samsung uses the same line for all of their phones with the same boards; the differences are in the ROM, radio firmware being tweaked a bit and carrier-locked status. Generally, U phones are sold "locked" and were done that way at the factory, 1U can be locked at the factory, a dialer command I don't know or care to, or based on the first SIM installed (the carrier who sold it to you, similar to the Apple reseller flex policy used by every reseller except Apple unless it is financed through AT&T).