> you can use the Dropbox logo in marketing material to show your app's Dropbox integration, but it should never be modified and should always refer to our service. If you are using the logo on a website, it should link to https://www.dropbox.com.
So they're allowed to use it, but it must link to the Dropbox website. It doesn't so it's actually violating the guidelines.
From the Dropbox brading guide ( https://www.dropbox.com/developers/reference/branding ):
> you can use the Dropbox logo in marketing material to show your app's Dropbox integration, but it should never be modified and should always refer to our service. If you are using the logo on a website, it should link to https://www.dropbox.com.
So they're allowed to use it, but it must link to the Dropbox website. It doesn't so it's actually violating the guidelines.