I would not WANT bash to execute the command just because I pasted it into the terminal. I want full control of when and how commands are executed, and often I use pasting a command followed by other segments of text related to how it is to be executed or as part of a string of commands I am building. Having the command execute as soon as it is pasted into the terminal would be at times foolhardy, especially if you were doing it as root.
Just as an FYI:
If you paste a command in when the paste includes the newline, like you can get from a terminal line or from copying from a web page for example, the command will execute.