What makes you think it's a best practice? It may be usual, but when it comes to getting people to notice (and therefore, do) things on the web, "usual" isn't

What makes you think it's a best practice? It may be usual, but when it comes to getting people to notice (and therefore, do) things on the web, "usual" isn't usually that good. The rest of your comment is a straw man fallacy. Of course you can't test everything. But, you know...it might just be reasonable to test the conversion button on your landing page, especially when someone who does a lot of testing tells you that it has a big impact. Because that's a more informative prior than "we should just use the default behavior".