not entirely true.
If you were fitted in a shop and are absolutely sure you know your brand, model and size, you can buy online (kind of a dick move, though)
the other exclusion to the rule is: if you have experience/know how to fit snowboard boots and you have money to play with. You can then order the model(s) you're interested in and, when they don't fit, send them back and reorder. Repeat until you find the perfect size. Unless the website has prepaid returns, you will be paying return shipping, so any savings will be gone.
Yeah, that's why I am buying from Zappos, best return policy ever.
Also, some small towns just don't have a lot of snowboard shops so if u want a bigger selection... U have to go online.. I went to all my local shops... 4 of them and non of them had Burton driver x boots. Only one shop had vans boots and they were all out of my size.. And I live in a pretty big town (Las Vegas)