A using statement affects the global scope, so it doesn't make a lot of sense to evaluate it in local scope. That's why it's not allowed. The eval function evaluates in global scope, so it can execute a using statement.
In general, though, if you are eval'ing a using statement, you should probably reorganize your code to do the using directly in global scope. For example, put your code into a module if you want to keep the using statement isolated from other code.
