Hmmm Right, whatever.
Firstly the hitsniffer code is javascript. It cannot magically work on a page on which it is not installed. The script needs to be installed on each page that you want to track.
Here's how that is described in the hitsniffer website :
Input this code under your closing </body> tag, This must be on every page of your site
Fairly evident.
If you want to put the code onto every page of a Zen Cart site then what you do is put it into tpl_main_page. If this is done correctly then it will not crash the page. This is pretty standard stuff and applies to many scripts not just hitsniffer. For instance it is where google analytics is put on many sites. It is a standard technique not only on Zen Cart but also on wordpress, drupal, joomla and lots of other php based sites.
Here's how the hitsniffer website describes that:
If your website have a footer.php , footer.asp , global.footer.php or something like it, you can simply paste tracking code before </body> tag and they will usually show hit sniffer code on all your pages automatically.
While Zen Cart does have a tpl_footer.php file it is better to put it in the tpl_main_page.php file. This ensures that the script is directly under or above the </body> tag and is below the parse times and lower banner on a standard install.
If done correctly this will not cause the site to crash or display a blank page. Most likely if that result is achieved it means that the script has been pasted inaccurately inside <?php ?> tags.
Actually the script will probably work wherever you put it on the page. The 'best practice' is usually to put it as low down in the page as possible. Although we can have a long discussion about the pros and cons of that. Or not -- and just do what the hitsniffer website recommends. (Notice how the advice from hitsniffer is slightly inconsistent about whether it goes above or below </body> )