That's essentially why people think its not-free. In essence, it pretends to give you the right to do something, but puts impossible to meet requirements so that in pracitise you cannot do the thing.

In essence, the license says you cannot use it as SASS software, but they didn't want to outright say that, so they did this instead.

You only need to do that if you operate it as a service that you sell or give away to others (rather than just running your own internal redis instance)

So yes, this basically prohibits anyone but redis from operating it as a service, unless you write your own operating system for it to run on. (Although presumably they will sell you licences or similar to operate it as a service)

That's the point! It's impossible to comply with it. If it were an open source license, it'd require every component to be released under an open source license.

That's why it's dual licensed.