¿Por qué el núcleo de Bitcoin crea transacciones bloqueadas por tiempo de forma predeterminada?

TL;DR: El objetivo es desalentar las reorganizaciones y aumentar la privacidad.

En wallet.cpp, justo arriba de donde se establece nLockTime , dice:

// Discourage fee sniping.
//
// For a large miner the value of the transactions in the best block and
// the mempool can exceed the cost of deliberately attempting to mine two
// blocks to orphan the current best block. By setting nLockTime such that
// only the next block can include the transaction, we discourage this
// practice as the height restricted and limited blocksize gives miners
// considering fee sniping fewer options for pulling off this attack.
//
// A simple way to think about this is from the wallet's point of view we
// always want the blockchain to move forward. By setting nLockTime this
// way we're basically making the statement that we only want this
// transaction to appear in the next block; we don't want to potentially
// encourage reorgs by allowing transactions to appear at lower heights
// than the next block in forks of the best chain.
//
// Of course, the subsidy is high enough, and transaction volume low
// enough, that fee sniping isn't a problem yet, but by implementing a fix
// now we ensure code won't be written that makes assumptions about
// nLockTime that preclude a fix later.

txNew.nLockTime = chainActive.Height();

// Secondly occasionally randomly pick a nLockTime even further back, so
// that transactions that are delayed after signing for whatever reason,
// e.g. high-latency mix networks and some CoinJoin implementations, have
// better privacy.

if (GetRandInt(10) == 0)
  txNew.nLockTime = std::max(0, (int)txNew.nLockTime - GetRandInt(100));

Como notó Martin Habovštiak, el número de secuencia se establece en MAX-1para cada entrada. Está en el código justo debajo de la sección de código mencionada anteriormente .