You still have a variance penalty. The point is that at the margin you should have linear utility, especially if you can rebalance to other strategies.
The arbitrageurs make money, ergo the LP could just decide to run the arbitrage against themselves. If they don't, it's because of risk considerations, ergo utility factors in for your argument. Not a red herring.
It is indeed possible that this is true; this is simply a claim that small but nonzero fee is generally optimal but we make no claim about its total utility. (Though the PDE and the explicit result we give depends on having a quadratic cost function.)
I think that having a small but nonzero fee is only optimal *if* you have a nonlinear utility function; with a linear one I think infinite fees are optimal