A heavy handed and crudely written attempt at trying to make a statement about how pollution harms the environment by the looks of it.
I feel like having to play devils advocate here, but this a realistic scenario and part of a world wide problem involving the massive amount of waste produced by the modern world and it's industries.
Besides the possibility of the bag being simply carried by strong winds through upper atmospheric layers until it landed in the ocean, there is also the aspect of waste disposale companies and their treatment of things like plastic waste.
Plastic bag is thrown in the trash in las vegas -> Plastic waste is collected by a big waste disposale company -> Throws all plastic waste into a big container -> Multiple scenarios:
1. Company just illegaly disposes of the plastic waste in the ocean.
2. Company ships the plastic waste to poorer countries for further disposale.
2.1. Plastic waste is not properly stored and partialy falls into the ocean.
2.2. Workers don't care if waste is lost so it flies away and into the ocean.
2.3. Subcontractor disposes of plastic waste in the ocean for cheap.
2.4. Company just throws waste on a large dumb in developing countries, where it will remain unsupervised and likely filed through by poor locals, causing it to be scattered to the wind and ending in the oceans.
People have found plastic waste in the ocean they can date back to be more than 40 years old and from the other side of the world, because the writing on it is still intact.
But intact plastic isn't the worst problem, it's when it's grinded down to fragments which fishes, oceanic animals and fish eating birds swallow (via the fishes), causing their stomachs to be sealed up or they get it into their mouths and nostrils where it gets stuck or creates cuts. There are pictures of sea turtles with plastic pieces in their nostriles as if someone rammed a long nail in.
Micro-plastics (plastic waste grinded down to microscoping size) are even worse.
So the scenario of a sea turtle dying from plastic bag that comes from Las Vegas and somehow ended up mostly intact in the pacific ocean is actualy a real scenario because of either the interconnected weather system or the global waste transport and disposale industry.
However this story is so crudely written and trying to hammer the reader over the head with this real world issue that it runs the high risk of doing more harm than good for anyone to take it serious.
Especialy with the questionable moral displayed here, by having a previously innocently written young mutant character perform first degree murder for lower reasons.
"Don't work in a grocery store that uses plastic bags or a mutant girl comes in and stabs you!"
And just because the victim is middle aged white guy who was presented as "insensitive" it's not going to make the crime any less extreme. What if the guy is a loving family father who together with his wife had adopted 3 orphans from the Sahel region in africa? Who is going to tell them that their husband/father got killed by a mutant over a dead sea turtle and a plastic bag one of his customers threw away?
Edit: Also thinking about the 40 year old plastic waste example above. The bag Nature Girl finds could have been from the 80's long before the clerk actualy started working in Owen's shop.