Not necessarily the policy for all departments, and also not necessarily protective.. The defense expert witness in the trial of the officer in the New Year's Day, 2009 BART police shooting (where the intention was to use a TASER on the suspect but the suspect was shot) pointed out that in six prior cases where firearms were mistakenly drawn instead of TASER's, the TASERS were carried on the same side as the duty weapon. In the BART case, the officer indeed did have his TASER mounted on his (non-dominant) weak side - opposite of his firearm - but in that setting the TASER is still set up for use in cross-draw mode with the dominant hand - the same hand that would be used to deploy the duty weapon. Of course in this (2009 BART Police) case the mounting of the TASER on the opposite side of the duty weapon didn't prevent unintentional firing of the duty weapon with fatal results.
Another reported technique to reduce errors in which weapon is being deployed is strong (dominant) side mounting of the taser on the duty belt in a cross-carry configuration - to make it very difficult to deploy the taser with the strong side (dominant) hand as opposed to easy to deploy the service weapon with the strong side hand. I don't know offhand of any evidence of how well this works (although I'm sure it's out there).
I see this as a human factors issue. Compare to retractable gear aircraft (at least small ones, i don't know much about large aircraft) - the gear lever usually has a handle that feels like a small tire and the flap lever has a flat lever similar to a flap. That's a human factors countermeasure to help dissuade activating the unintended control.
On the other hand, the M-26 model police model TASER is shaped like and feels like a pistol - presumably to make quick deployment and use easier. It may be that this was a bad design decision, and I'm sure it will be discussed in the wake of this incident.
On the deal with a 73 year old reserve officer - coincidentally a high-value donor to the department - in an sting operation? I'm with KState_Poke22. WTF? That just looks bad.
References
https://www.policeone.com/officer-shootings/articles/1772254-BART-shooting-raises-issue-of-TASER-confusion/
https://www.californiabeat.org/2010/06/28/mehserle-defenses-use-of-force-expert-falters-during-cross-examination