First things first: Datasheet links are great. Don't ever worry about linking to a datasheet when you're doing it to allow readers to get more information on a part. Digikey currently reports that there are 2.3 million parts in their database (with some duplicates for Digi Reels and part variants), and Octopart reports 1.9 million parts. No one can keep track of all these parts. Datasheets are invaluable tools for understanding a question or answer.
Second, I haven't seen any problems with major companies. If you're linking to an Atmel, National, or Xilinx datasheet and you're not a member of that company, then you're just fine.
The only times that this is suspicious is when you're referencing products from a specific distributor. Even then, if the only datasheet you can find is hosted by a distributor, and you're linking to the PDF, you're probably OK. However, please don't link to the product page on the distributor website. For example, as much as I like Sparkfun, I wouldn't link to http://www.sparkfun.com/products/10953 when referencing this accelerometer. The PDF link at http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Sensors/Accelerometers/MMA8452Q.pdf would be a good deal better, especially since the uninitiated probably wouldn't associate that cloudfront.net subdomain with Sparfun. The best place to link to would be http://cache.freescale.com/files/sensors/doc/data_sheet/MMA8452Q.pdf; the manufacturer website.
Finally, remember our promotion rules:
May I promote products or websites I am affiliated with here?
Be careful, because the community frowns on overt self-promotion and tends to vote it down and flag it as spam. Post good, relevant answers, and if some (but not all) happen to be about your product or website, so be it. However, you must disclose your affiliation in your answers.
If a huge percentage of your posts include a mention of your product or website, you're probably here for the wrong reasons. Our advertising rates are quite reasonable; contact our ad sales team for details. We also offer free community promotion ads for open source projects and non-profit organizations.
This is really an ethical issue, not a legislative one. If you're reading this as a member of a company with products you want to promote, take the high road, don't try to subtly advertise your company by posting links in lots of answers.