At least for the Huawei sticks (e.g. e220) they are more than just an AT-compatible serial protocol device. The same USB device tries to support two different serial channels as well as a CD-ROM device. The CD-ROM part contains a driver which is automatically installed when you plug it in to a Windows- or Mac computer.
A Linux kernel without support/patches for those USB sticks will get confused by the different personalities and may decide to only show your stick as a CD-ROM (which it can't even properly support). But with patches, or the latest kernels it will handle it as either one or two serial channels. Only one of them is really important because that's where you send your AT commands. The other serial channel is for monitoring and may need some additional software. The CD-ROM part can be completely ignored as it's useless for Linux anyway.