Proper web design should follow cross-platform standards and implementations.
To be blunt: says who?
A lot of people and most organisations aren't making websites as a charitable exercise. They're doing it with a goal in mind, such as bringing in money directly or indirectly, or raising awareness of a cause they care about.
Whatever that end goal is, they need to use the web to communicate effectively with their visitors. If those visitors are mostly using one particular browser and they can achieve better progress towards their end goal by optimising for that browser, that is what a lot of them are going to do.
I don't think this is necessarily healthy for the long term future of the World Wide Web, but I also don't think it's reasonable to blame people with a job to do for choosing the most effective tools available to do that job.
No such thing. A job, like an order, is not something that actually exists outside of the actions of people. Both the people giving orders and the ones following them remain responsible. They can pretend to leave the court room by dozing off but they remain in it for those who haven't, and what you seem to see as "putting blame" is simply pointing out what is already present and cannot be removed.
This is a very short sighted decision, apart from technical demo, designing a website to only work with a specific browser is actually costly, doesn't last and significantly limits your reach.
It's 2017, the era of evergreen browsers and living standards. As much as you or I may wish otherwise, anything built for the web today may not last and may have significantly limited reach within a matter of months. Every major browser developer has shown a willingness to cut off established functionality, even things that have worked for many years, if its suits their purposes. (Either that or they've abandoned older, non-evergreen browsers more or less completely.) The only standards that matter in web development today are the de facto standards of what the browsers actually do, just like the bad old days of IE vs. Netscape.
As I said before, this might well be bad for the long term future of the World Wide Web as a resource for society, but in this business you have to play the hand you're dealt, and the browser developers hold all the important cards.
To be blunt: says who?
A lot of people and most organisations aren't making websites as a charitable exercise. They're doing it with a goal in mind, such as bringing in money directly or indirectly, or raising awareness of a cause they care about.
Whatever that end goal is, they need to use the web to communicate effectively with their visitors. If those visitors are mostly using one particular browser and they can achieve better progress towards their end goal by optimising for that browser, that is what a lot of them are going to do.
I don't think this is necessarily healthy for the long term future of the World Wide Web, but I also don't think it's reasonable to blame people with a job to do for choosing the most effective tools available to do that job.