Antes in ring?
Anyhow, the basic idea is that you want to be able to win the maximum amount possible for any given hand. Thus, if there is an ante, just add that to your minimum buy in.
While the ideas are not the same in FL and NL, think of buying in for less than 12BB + antes as the FL form of shortstacking. Anyone who buys in for less alters the strategy that must be used (shortstacking in limit is impractical because the stack size relative to the short stacked buy in changes so drastically with any given hand meaning we can't double and jet because it will happen too often). However on the high end of buy ins, because the amount we can win in FL is capped every hand, it doesn't make any difference whether or not we buy in for 100BB or 12BB + antes because we can never win/lose more than 12BB + antes per hand so strategically, it doesn't make any difference assuming you rebuy to 12BB + antes at the beginning of every hand.
Oh, and just in case it wasn't readily apparent, the 12BB comes from 8 small bets from preflop and on the flop, and 8 big bets on the turn and river giving us 8*.5+8*1 = 4+8 = 12