Here’s why. When you install tile with the installation products that you either bought or accepted from another party, and you install tile over a substrate that you have prepared or accepted as finished, you are asserting that your installation will perform as intended. Both tile and installation materials manufacturers all require their products to be installed per industry standards and per their own installation instructions. So if you don’t follow industry standards or manufacturers’ instructions and something goes wrong, you are on the hot seat. Industry standards are created by a consensus group of experienced tile installers, manufacturers, scientists, and other industry experts in order to prevent installation problems.
So if the GC or the client tells you to install tile without the required movement joints, or to install over a substrate that isn’t properly prepared or sloped, then you better walk away from the job or get a statement in writing to cover your behind. The written statement should outline how you were asked to deviate from industry standards, and that you did not recommend any of the deviations, and that the parties involved indemnify you from any resultant damages. Make sure they sign it and keep a copy tucked away in your file for a rainy day when that phone may ring!