Both WestLaw and Lexis provide some additional blandishments which distinguish their stuff from the court's original copy and from each other; each works up (independently) headnotes summarizing the issues, a table of authorities listing the precedents and statutes mentioned in the case, and a list of subsequent cases and law review articles mentioning this decision (until about 3 years ago, both WestLaw and Lexis were contracting with Shepard's Citators for this data, then Lexis bought out Shepard's and WestLaw cooked up its own equivalent, West Cite, using a very complicated computer program to constantly scan the entire database for mentions of the case being studied). They also do some proofreading and expand on case citations.
It's true that court decisions are public domain and so forth. But - although a court might give you a copy of one of its decisions if you specify what you want - I don't think any court will put you on a mailing list to receive ALL of its decisions, for nothing. And a court won't find for you an unnamed decision dealing with a particular topic. You are paying WestLaw and Lexis for that service.