If you use a sample in your beat, then you are using another persons copyrighted material. And it's likely that you haven't paid for a license to use that sample. Which means that there is a potential lawsuit in the works if the owner of the copyrighted material decides to enforce their rights. Do you think Soundclick wants to be in the middle of that lawsuit?
And even if you obtained a license to use the sample from the owner, then there is a performance royalty due for the streaming of the music over Soundclick (or any site for that matter), as well as a mechanical royalty due for the sale of that music, both by you AND by whoever records on the music. Who's going to pay that royalty? Soundclick? I don't think so. They'd be losing money all day paying royalties for music that their users are uploading.
To boot, nobody wants to release a beat where they have to pay royalties up the yin-yang... to the songwriter, to the recording artist, to the sample owner(s). So you shoot yourself in the foot by making sampled beats, with no hope to go any further than selling $20 leases to no-name artists under the radar.
You see the problem with samples?