He could have forced his son to become a Maester instead of taking the black. Same end result.
Randyll is from the Reach; Oldtown is practically next door, there should be some affinity here. He might not even have needed to twist Sam's arm too much; his son is clearly well suited for te Maester's life. His wife would doubtlessly have been much happier.
So why force the black?
First possibility is the expense. I have not read anywhere that some fees should be paid to the citadel so the accept your child, but it seems likely. Unlike the Night Watch, the Citadel doesn't seems to have lands or any way to sustain itself. I have to assume they get some income at some point, a entry fee seems likely. And if so, mayhaps Randyll Tarly did not want to pay it on behalf of Samwell.
Or perhaps he just hated his son so much he'd rather see him at the wall in the hope he suffers and dies.
Or hey, maybe both.
What a jackass either way.
Edited by Mal Malenkirk, 12 July 2014 - 08:42 PM.