Why not bundle it with the mother module?

I explicitly removed this functionality from Webform because I wanted it to be a separate project. This one feature alone causes a good quarter of support requests and "bugs" because things don't function properly when adding in who-knows what code. If you used this functionality, of course you can simply install this module and everything works just like before. I just don't plan on providing any support for anyone that uses this module.

but even Drupal core ships with this option

But even Drupal core discourages its use, that's why all PHP functionality in core was broken out into the "PHP Filter" module in Drupal 6 (it used to be part of Filter module itself). I'm sure if we had the choice, PHP filter would be removed from core (maybe it will be in Drupal 8). I'd also like to note that there is no need for having textareas to execute code, since you can build anything in a much more secure manner through a module.