SendSpace is a file-sharing site, not a P2P one.
It's intended, for example, for people who might want to create a 20Mb video of their wedding and then distribute it to all of their family members. They can't e-mail it because many e-mail services have a maximum attachment size which won't handle 20Mb, so they upload it to the SendSpace server and provide the link to their family members.
All files are private and only available to those people who've been given the link.
This is from the SendSpace FAQs page:
Q: Can I search sendspace for files?
A: No. The uploader determines who has access to their files. For copyright and privacy reasons we cannot provide links or a search facility.
Source:
http://www.sendspace.com/faqs.html
P2P networks effectively get round copyright laws because files aren't held on any one server. When you use a P2P network, your PC is effectively connected direct to someone else's without the copyrighted material ever passing through a central server. It's still illegal to upload or download copyrighted materials but, because there's no central server, it's extremely difficult for the copyright owners to take legal action against anyone.
If SendSpace permitted people to use their server for the distribution of copyrighted materials, they would immediately face legal action against them for breaching international copyright laws. That's exactly what happened to Napster, which was originally closed down before being reborn as a legal, fee-paying music download service.
Chris