Merc Tony was going to reinforce Sargoth, he moved during the night to avoid being attacked - I would have also in his place.
But since the Forces of Freedom attacking Sargoth wasnt interessted in yet 3 more sieges they had to somehow attack Tony, night or day. And if Tony wanted to bravely fight his way thrue enemylines and reach the castle - he could have done so at another hour during the day. Like Zlisch said earlier, the window for an attack wasnt big.
But fact is, the only area where Mercs actually can move freely is between Ismirala Castle and north up to Alburq - its like a little merc-reservation - outside of that area its game on so they have to move nighttime.