Because in shared hosting environment every users shares the resources some preventive software must be placed to prevent abuse. Bluehost have cpu quota/usage limit placed to prevent abusive users to take down the server. This have been very restrictive in previous years and lots of users have complained to get cpu quota errors:
“This Account Has Exceeded Its CPU Quota”
The software that calculates the usage limit have been updated (January 25, 2008) with the newer version of the software that changes the way how CPU usage is calculated on the servers. This should reduce the number of people that run into this problem by 90%. You can read more about the software update at Matt Heaton president of Bluehost, Hostmonster and Fastdomain official blog: http://www.mattheaton.com/?p=119 .