A command scope variable is left assigned after executing the command

Question

Let's check the following script:

#!/bin/sh

func()
{
  echo "var: $var";
}

var="val" func

echo "after executing the command, var: $var"

Its output is:

var: val
after executing the command, var: val

The var variable is left set after executing the command.

Is it correct behavior? I thought that command scope variables are set just for a single command.

If I use bash (set #!/bin/bash in the first line) instead of sh then setting a variable for a command doesn't affect the rest of the script:

var: val
after executing the command, var: 

In the busybox implementation sh and bash work equally: the variable is left assigned after executing the command.

To workaround the problem I use parentheses when executing such commands:

(var="val" func)

This way the var variable isn't touched in all shells.