They had to choose what class of user was going to be optimized and which class was going to get an extra step or two.
When the software engineers were sitting around the table they decided that 90% of the users want clamshell mode when they close the lid with a monitor connected. Those that don't want clamshell will have to live with an extra step (turn off or disconnect the monitor first). Pretty simple they thought, but I guess they did not realize how annoyed 0dev would be that his/her needs were not placed above all others
Now this is a area where perhaps windoze machines have an advantage. Windoze includes a control panel to select what should be done when the lid is closed (can be different for battery or powered). It is a PITA to find and figure out, but there. Perhaps MS has a patent on the feature and won't let Apple use it..... I wonder if there is an app for it, there are several clever utilities about, like those that dismount external drives upon sleep so you can grab an run in the AM and not get that annoying disk was not put away correctly notice. The same utility searches for connected drives and remounts them when the laptop wakes.