default shell for remote commands through ssh

What you are seeing there is the local login shell.

ssh user@host "echo $SHELL"

With the above, $SHELL is expanded before ssh is run because it's enclosed in double quotes. So on the remote end you're running echo /bin/zsh instead of echo $SHELL.

Use single quotes to avoid $SHELL being expanded locally.

ssh user@host 'echo "$SHELL"'

See BashFAQ 96 for more on this.

You or someone else must have configured your remote host with zsh. To switch back to bash use:

chsh -s /bin/bash

on your remote host.

Though as an aside I would recommend zsh it's an awesome shell with lots of very useful features, and having it enabled doesn't stop you from being able to use bash

#!/bin/sh 

will still run your scripts under bash.