Controllers are expected to issue ATC clearances conforming to what?

The expectation for controllers to issue ATC clearances that conform to standard aircraft operational capabilities is rooted in ensuring safety, efficiency, and compliance with regulatory standards in the air traffic control system. Clearances must consider the operational characteristics of various aircraft, such as their performance limitations, maximum speeds, and climb rates, to provide instructions that are realistic and safe for pilots to follow. This alignment helps mitigate risks associated with overloading an aircraft's capabilities or putting it in a potentially hazardous situation.

In contrast, last-minute amendments, while they may happen, should ideally be minimized to maintain clear and effective communication between controllers and pilots. Pilot preferences can vary significantly and might not always align with safe operational standards or air traffic management objectives. Weather conditions are indeed a crucial factor in air traffic control but do not directly dictate the standardization of clearances; instead, they may influence adjustments to traffic flow and routing as necessary.