Once upon a time French was used even in the Dutch speaking parts, in government, economy, high society. That was long before the internet got popular though. We've had a long fight to get rid of French in Flanders. Dutch is the only official language in Flanders, and it's the language people speak (except expats, or migrants who haven't learned the language yet).

So when companies still assume Belgium == French, it's not only wrong, but it comes across as very condescending. It feels like they haven't outgrown the times where Dutch was suppressed in favor of French.

And why? If a company wants to use only one language for the whole country, it's better of choosing Dutch (as we indeed account for about 60% of the population). Many of those companies do have a perfectly good Dutch translation, which they use for their site when viewed from the Netherlands. Even if they don't, I much prefer English over French.