There are a number of articles that claim that just installing the demo installs securom onto your machine. Here is another. How many do you want?
http://www.privacydigest.com/2007/08/25/bioshock+demo+installs+securom+service+gamingbob+com
A quote from the article: "What 2K has failed to address is the the SecuROM service installed on your computer when installing BioShock, which is also included in the demo."
I am not saying that it does this definitively all I am doing is reporting on it. I also have read that people that dl from Steam have not reported this problem.
Here is a quote from Securom and a response regarding this question:
" SecuROM DOES NOT USE any root kit technology in its implementation. SecuROM Product Activation does place a folder and registry keys on your system. These folders are used only for storage of license information and information to assist with disc authentication. Please do not modify or delete these files. Tampering these files may result in authentication issues."
And a typical response:
"Oh, I tampered all right. I tampered, and I’d do it again. And AGAIN. I don’t know about you guys, but AIDSware or not, I’m very selective about what kind of software and registry keys are installed on my machine, especially when said software is notorious for wreaking instability - no thanks."
Finally what is mentioned in a number of places is that once you uninstall the game or the demo if the securom reg keys were installed they are not removed with the uninstall.