I have written content for many, many businesses who sell branded products. I always ask if they have permission to use the info on the manufacturers website. Sometimes they do and sometimes they don't. When they don't I either get the permissions myself, ask them to acquire them, or find another road to travel.
Copy/paste or 'rewriting' from another publication (which is call a 'derivative work' under copyright law) is NEVER okay without the permission of the copyright owner. It is illegal as well as unethical. The copied website may reach out to you with a request for removal. They may also reach out to your webhost to have your website taken down, or they might just take you to court depending on the loss they feel your plagiarism has incurred for them and how deep their own pockets are.
Also to the point, if I want news or info about (example) Apple's new iPhone, why on earth would I get it from your blog when I can get it either from Apple or from credible media that has gone to the trouble and expense of researching the subject and requesting Apple's permission to use the information they have found along with their trade and/or service marks?
If you want to do well by your visitors -- and you should if you want to be successful -- then find original topics and write original content, acquiring the permissions you need if and when you need them. Writing for the web isn't any different than writing for print. It's a job. Do it right to be successful.