Yeah it used to be quite hopeless unless you opened the link in a tab that had already loaded WhatsApp web. Now however, if you click the green button that says "CONTINUE TO CHAT" on the initial screen, it'll load a page with an option to download WhatsApp and another that's titled "use WhatsApp Web", just click on that one. Unfortunately, it will load a new instance of WhatsApp web in the current window even if you already have another open in another tab.
Well, in Zimbabwe because of the large user base WhatsApp got when it initially arrived (due to reduced charges, SMSs where about 15cents a pop then) around 2011-2012, the ISPs have effectively made it the only way to communicate economically by providing a "bundle".
Basically they offer you a drastically reduced price to only access WhatsApp, mostly text message without media at that. Compared to the data plan that can access the whole internet it is very very cheap to use just WhatsApp.
The cheapest WhatsApp bundle on the most popular network (Econet) is currently at around 51cents to text the whole week (65MB of data) vs the $13 for 8gigs. Granted the 8gigs is for a month but $13 is a big amount in this part of the world, and with regular data there's the aspect of discipline to not use all that data on youtube and other data hungry apps/websites.