If a third party has a digital storage device which contains evidence of a crime then that storage device (and computer the device is in) can be seized by law enforcement as evidence.

1. Does that happen if the evidence is a file uploaded to a cloud storage service like Dropbox, Google Drive, etc? My intuition says that in cases like that the LEOs aren't going to track down and seize each and every storage device that the file might be scattered across, but that's just my intuition.
2. If it doesn't happen to cloud storage services, then why? LEO discretion? The law considers the services trustworthy enough to not tamper with the files? Or what?

If a third party has a personal computer which contains evidence of a crime then that computer can be seized by law enforcement as evidence, rather than the data being copied from the computer and the computer being left with its owner. However, as exampled here, when evidence of a crime might be found in a cloud storage account, law enforcement asks for a copy of the data rather than physically seizing from the cloud service the storage devices on which the data is stored.

So why the difference? Is it merely a matter of practicality, or is there some law requiring physical seizure of the computer in one circumstance but not in the other?