Why would IQF shrimp say "do not force thaw under running water"?

Three possibilities that I can think of:

1. If the water weren't cool enough and they were big shrimp, it could raise them to a temperature at which bacteria could develop.
2. If the water were actually warm or hot, it could start to cook the shrimp.
3. If the shrimp were defrosted directly under water, not in a bag, they could absorb some of the water and the texture would become mushy.

So of course Costco could write "If you defrost the shrimp under water, make sure they're in a bag and the water is cold." But perhaps they feel that people are more likely to read a warning than an instruction, so their current wording is safer.

Just guessing, anyway, but a quick Google search doesn't turn up anything more likely or useful!

Update

Jeff Axelrod commented (much thanks) that the Costco IQF shrimp come with most of the shell removed. In that case, "DO NOT FORCE THAW UNDER RUNNING WATER" most likely means if you do it, you'll end up with "shrimp pulp" with some of it even down the drain. The shrimp can absorb the water and get mushy and disintegrate. This, I have seen in our kitchen and tried to erase from memory.

This also means the trapped bacteria under the shell is less of a concern.

the original answer/caution regarding safety:

Besides the fact that the shrimp can absorb the water, get mushy, and fall apart under running water, it could be a safety issue.

It may be possible that the pressure from running water will force or push some bacteria that would normally be on the on the outside of the shells (and destroyed) in and under the shell (e.g. between the belly and the shell) where it may be shielded from heat and not fully killed during cooking.

Bacteria going from outside of the meat to inside has caused Costco a lot headache. In the meat case, it was forced in mechanically by the needles of the tenderizing machine. They might also be hedging against a similar thing happing with the shrimps and running water.

If you thaw them in a tight freezer bag placed in water (even running), it should be safe.

The biggest loser when IQF shrimp are "force defrosted" is diminished flavor profile and loss of texture (or mouth feel - they should have a certain resistance to the bite). There is no danger of any food safety issues when force thawed. The very best way to thaw any IQF shrimp of any size and style is to leave them in the bag, put the bag in a bowl, and leave them in the refrigerator until they are defrosted - typically over night. If the shrimp are mushy or tough after defrosting overnight, it means that they were chemically treated (usually with sodium tripoly phosphate) or they suffered from time and temperature abuse during processing and shipment.

While thawing items under cool running water is a quick method done in just about every restaurant that uses frozen items, it has a side effect of giving you a less than desired end product. Basically the quality goes down. Water is absorbed into the item, the flowing water can damage the item texture yada yada yada. Best to thaw in the fridge as a rule for best results or as stated in other posts, sealed in a baggie and soak the bag so you don't get the water contact.

That said, when dealing with thawed shrimp/prawns, I've always found that soaking them in an ice cold saltwater bath(seawater like) for 5-10 minutes prior to cooking helps to firm up the flesh, add flavour(yes, I know it's really just salt) and give your prawns a little bit more "pop" when you bite into them.

I'm thinking Costco is just trying to ensure the end product you use will be of good quality.