Sometimes but not usually it's the retailer who initiates the stalking.
If you look closely, you'll find most big online advertising companies are using a few, but very similar principles. Google, Yahoo: it's how they make money, however usually through a daughter company to hide the link. It's the traditional approach to targeted advertising.
It becomes evil when sites like Facebook get involved, here the idea is they can read your beacons and spread them to your friends on the basis that they might want to buy the product they saw you buy. Not to mention the opportunity to enrich the beacons with target demographics like sex, age, location, etc...
This type of advertising is done on the principle that people who buy something will often buy similar things, accessories, or at the least that their friends will.
It is of course very backwards but just like spam, those few people who actually buy after being prompted with these ads make it all worth while. (And that doesn't mean click on the link but buy something from that site/brand within a few days or weeks.)
Fortunately, AdBlock and similar options do a very good job of blocking that content. Unfortunately some ad revenue based companies have become smart enough to break their usability if the code sent by the ad tracking is missing. So sometimes you will need a "GreaseMonkey" to get around that but it can be done.