Shouldn't "once" be followed by PAST?

[Odessa Dawn]

In other words, Obama is motivated to shield Bush officials from accountability for their crimes in the hope that once Obama leaves office, he, too, will be gifted identical immunity from the rule of law.

Source of the context:http:Obama's justice department grants final immunity to Bush's CIA torturers | Glenn Greenwald | Comment is free | guardian.co.uk
Source of grammar rule:Learn English Online - English grammar and usage - correct usage of once

Should I pay attention to English grammar rule or not?

[riquecohen]

"Once" is used here as a conjunction. Return to your link and see the definition under "Conjunction." Also, look here. once - definition. American English definition of once by Macmillan Dictionary
It is frequently helpful to consult more than one source.