What's up with companies asking for personal information that they don't really need, but they ask for it anyways?

Apparently to top up your prepaid mobile phone, you need to provide them with your email address. What's up with that? I left it out, which they promptly went, "wah! we really need it!" ... so when you fill it in, they provide this information on the next webpage anyways. Thanks guys, for letting me know what frigging retards you are, because you just told me you don't really need it.

By the way, I don't like the SMS advertisements they send me which I have no way to opt out of. I have been thinking of switching, but it's a matter of finding who will piss me off the least...