I understand this is incorrect, but what is the reasoning why its not allowed to go against gravity before it comes back down? as far as safety and functionality goes
I would think that if the TPR valve was leaking, that the result would be water accumulating in the line and subsequently back contaminating the water in the tank.
That, and it’s not per manufactures installation instructions.
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As Kevin mentioned, it becomes a water trap.
Not only can bacteria grow in the trap, it could freeze, it could degrade the TPR, it could created enough back pressure during a TPR activation that the tank evacuates in some other direction or the pipe blows off or any number of thing.
Are the chances of that remote? Sure. When it comes to a hot water bomb sitting next to a habitable space, you want the safety defects to get as close to zero as possible.
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Thank you, Kevin, so I did a bit more digging as far as back contaminating goes, I was under the assumption the hot water of a water heater would kill any bacteria. but it appears that I am incorrect. thank you again that about answered my question.
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All replies above are good and well, but the reality is, most TPRV’s will “Drip water” at the first stages of failure. This is your first RED flag warning that something is wrong!
Have you ever seen a small wet spot or stain beneath the end of the drain tube? What did you say about it? 
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Or gushing like this 
The homeowner must be perplexed by their high water bill. This has been going on for a long time.
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I’ve seen water heaters leaking from the TPR valve pipe a small hand full of times, have any of you seen it actually burst out? just curious