typical reasons tile installers object to urethane or epoxy grouts: they've never used them; the grout sets up much more quickly; the extra grout must be cleaned off the tiles more quickly (or tiles get a haze); it's a much longer delay before the homeowner can get it wet.

a backsplash, especially behind a sink, i would categorize as "Limited Water Exposure" (not wet, but not dry either). for that reason, i would lean toward using an epoxy or urethane grout for the backsplash. these grouts can be made to absorb 100 times less than cementitious grouts. they don't stain and don't have to be sealed every year. but they are more expensive (materials + skill), too.

if your installer still balks, find another tile installer with experience using these grouts.