Wow, didn't expect this from WhatsApp. I wonder if we can hear it from their side to see if they have a specific reason. Also, are they blocking any other messenger links i.e WeChat, Line etc or just Telegram.
The current version of WhatsApp is clearly trying to make it difficult to leave the ecosystem. You can see that when you want to copy text in Android. If you copy it you can only send it to other WhatsApp users. In order to copy it somewhere else you need to use the share feature. In Telegram and others you can copy paste easily into other apps.
but they had to know that people would go dig through the sourcecode and find out that it was not just some random glitch, but a deliberate action on their part
FB is too large an organization to survive long when competition is eating a huge chunk of their services. And especially so, if the competition is someone like telegram who is both free and open source.
Their decision to purchase WhatsApp turned out to be very ill-timed. WhatsApp was at the peak of their popularity when they did, but that peak was only because of ignorance of the masses - ignorance about glaring holes in WhatsApp security, and ignorance about significantly better alternatives like Telegram. But in the age of Internet, ignorance hardly lasts long among the masses, of all people, Zuckerburg should have known this!
When FB bought WhatsApp, they estimated that the user base only had a useful life of seven years.
That was one or two years ago now and I'm not sure if the estimate has been updated, but it would seem FB is running out of time to convert the user base.
Assuming the source code posted in the article is the only code doing the blocking, it's only blocking links where a domain or subdomain is "telegram", although the regex could easily be altered to block other links, too.
It might also be due lobbying given that Telegram was recently in the news for being a popular medium of ISIS communication (not for the attack - as we know by now, but for its public channels)