Amazon.com Inc has a promotion for US shoppers on Prime Day, the 48-hour marketing blitz that started Monday: Earn $10 of credit if you let Amazon track the websites you visit.

The deal is for new installations of the Amazon Assistant, a comparison-shopping tool that customers can add to their web browsers. It fetches Amazon's price for products that users see on Walmart.com, Target.com and elsewhere.

In order to work, the assistant needs access to users' web activity, including the links and some page content they view. The catch, as Amazon explains in the fine print, is the company can use this data to improve its general marketing, products and services, unrelated to the shopping assistant.

The terms underscore the power consumers routinely give to Amazon and other big technology companies when using their free services. In this case, Amazon gains potential insight into how it should tailor marketing and how it could stamp out the retail competition.

"This data is often used for training machine learning models to do better ad targeting," said Bennett Cyphers, a technologist at the nonprofit Electronic Frontier Foundation. "But in the US, there aren't really restrictions on what you can do with this kind of data."

Amazon already has more than 7 million customers using its assistant via Google Chrome and Mozilla Firefox, according to data published by those web browsers. Other companies offer similar shopping tools.

While another technology known as tracking pixels shows Amazon information from visitors to roughly 15 percent of the top 10,000 websites, the assistant lets Amazon follow a smaller set of users from page to page, Cyphers said.

Amazon's combination of tools still pales in comparison to data collection by Alphabet Inc's Google, which has tracking pixels on most web pages.

Amazon did not discuss how it uses the data it gathers via the assistant for any unrelated purposes, but a job listing for an affiliated team known as Browser Integration Technologies says the group's influence "spans across advertising and marketing, pricing and selection."

"Customer trust is paramount to Amazon, and we take customer privacy very seriously," a company spokeswoman said, noting compliance with the assistant's privacy policy, which says data collection is for websites that users visit "where we may have relevant product or service recommendations."

The policy also notes that customers can disable certain features of the assistant, and that Amazon only links browsing data to an individual's account when the assistant is in active use.

US lawmakers have recently increased their scrutiny of Silicon Valley's data collection practices. A bill introduced in the Senate last month proposed requiring that big platforms disclose what information they gather from users and how much that is worth.

"When a big tech company says its product is free, consumers are the ones being sold. These 'free' products track everything we do," said Republican Senator Josh Hawley in a statement announcing the bill.

Amazon's Prime Day promotion offers a window into what it will pay for browsing data. For Prime Day 2018, it offered $5 off to new Amazon Assistant users who spent at least $25. This year it offered $10 off to those spending at least $50.

Amazon is also fine paying nothing for the data: New customers only get the $10 credit if they install the assistant from a particular landing page, if they are Prime members, and if they make their purchase via the assistant by Aug. 2.