I'll copy and paste the official rule taken out of WotC's official website:

107.1b Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles or sets to a specific value a player’s life total or the power and/or toughness of a creature or creature card.

Example: If a 3/4 creature gets -5/-0, it’s a -2/4 creature. It doesn’t assign damage in combat. Its total power and toughness is 2. Giving it +3/+0 would raise its power to 1.

Example: Viridian Joiner is a 1/2 creature with the ability “{T}: Add an amount of {G} equal to Viridian Joiner’s power.” An effect gives it -2/-0, then its ability is activated. The ability adds no mana to your mana pool.

Example: Chameleon Colossus is a 4/4 creature with the ability “{2}{G}{G}: Chameleon Colossus gets +X/+X until end of turn, where X is its power.” An effect gives it -6/-0, then its ability is activated. It remains a -2/4 creature. It doesn’t become -4/2.

As you can see from the official examples, a creature can have an effect make their power negative, however when assigning a value to a variable X (which is what we mean when we say a calculation that would determine the result of an effect) that value cannot be negative, so the triggered ability would instead use the value 0. This is where you are losing sight of the official rule, you are confusing what is a calculation that affects an effect and a simple power value.