It would mean that people with websites that have programs developed using gpl tools would stop using them if they didn't want to share sources. For some people, there are very good reasons to want that. In the long run, it would damage the spread of free software.
Furthermore, those tools' source is shared with the person who is using it; to website's owner and/or developer. The people using the website itself are not using the tool; they're using the website; while some of the output they get may have been generated by an open source tool, the overall organisation of that output is outside the tools' purview.