Why TrueSight Infrastructure Management (TSIM) java processes establish connections with remote cell(s)? How do I stop this from happening?

This is as per product design.
It is possible to stop it from happening to avoid resource consumption issues following the below steps:

1. Create a separate <process>.dir  files for the TSIM Rate, Agent Controller, Local Agent and Jserver processes that are establishing connection with remote cells. Make sure you do not add any remote cell entry in <process>.dir
2. Amend respective pw\custom\conf\<process>.conf file to read newly created <process>.dir  file.
3. Once changes are made restart process and check result.

For example:-  To stop 'Rate' process from establishing connection with remote cell(s),

• pnrate.dir is created without any remote cell entry in it.
• pnrate.conf is modified so that it would read newly defined pnrate.dir 

Add the following line under the Options section of the pw\custom\conf\pnrate.conf file:
Option=DCELLCONFDATAPATH= <path to the pnrate.dir file>
Option=DCUSTOM_MCELL_DIR_FILE_NAME=pnrate.dir

Similarly to stop 'Agent Controller' process from establishing multiple connection with remote cell(s),

• pnagentcntl.dir is created without any remote cell entry in it.
• pnagentcntl.conf is modified so that it would read newly defined pnagentcntl.dir 

Add the following line under the Options section of the pw\custom\conf\pnagentcntl.conf file:
Option=DCELLCONFDATAPATH= <path to the pnagentcntl.dir file>
Option=DCUSTOM_MCELL_DIR_FILE_NAME=pnagentcntl.dir

Similarly to stop 'Local Agent' process from establishing multiple connection with remote cell(s),

• pnagent.dir is created without any remote cell entry in it.
• pnagent.conf is modified so that it would read newly defined pnagent.dir 

Add the following line under the Options section of the pw\custom\conf\pnagent.conf file:
Option=DCELLCONFDATAPATH= <path to the pnagent.dir file>
Option=DCUSTOM_MCELL_DIR_FILE_NAME=pnagent.dir

After above changes restart process. (It is recommended to perform 'pw system start' instead of individual process restart)

*NOTE - Removing remote cell(s) from the pnjserver.dir file will result in not being able to connect to the remote cell(s) under 'Other Cells' drawer in the operations console. If it is still required to connect to those cells then consider not having Jserver configured to use a pnjserver.dir file.