First, perhaps it's wrong to model the agent's options as Left, Right, and Calculate. Instead, we should distinguish between genuine act options, Left and Right, and process options such as Calculate. Calculate is a process option because it's a possible way of reaching a decision between the act options. Alternative process options are, for example: trusting one's instincts, or calculating which option has the best worst-case outcome and then going ahead with that option. Arguably you can't go Left without choosing any process option at all. You have to either follow your instinct, calculate expected utility, or use some other process. So it's wrong to compare Calculate with Left and Right. We should rather compare Calculate with other process options like trusting your instinct. Doing that, we'd probably get the intuitive result that it's sometimes rational to calculate expected utilities (to varying levels of precision), and sometimes to trust one's instincts.

The claim "you can't go Left without choosing any process option" is overly strong and flawed. But a weaker version of that sentiment answers the problem adequately. Rather than saying it's impossible to go Left without choosing a process, just say that stumbling on the correct choice automatically is not a likely default, which makes calculation worthwhile.