There are probably restrictions, but I can't be sure, I have to do some research (that would be just for US right? and in case of developers), from the designer perspective I don't see any issue (I have worked for a NY start up with just and NDA contract signed, worked for a Finish Agency with a long term contract and NDA signed for over a year, my brother worked for a start-up in SF) no problems so far.
Connection is an issue indeed, but as was said in previous comments, 'Sometimes people from constrained countries HAVE to develop some skills ...' Finding Internet access is hard but not impossible. the most evident con it has, is that most of communication should be by IM chat, Skype is blocked and even if we could use it, or use a similar service, there is no enough connection speed to support this ... but as I said some my have this possibility as well if their payment is enough to cover a high speed internet access (i mean a faster speed connection, saying high is not quite accurate). But as I said I've been several years working like this.
And I know there might be some cons, but wouldn't save money be a pro? And not because I am Cuban, but there are very talented people living here, and there are more living outside Cuba that might benefit as well who knows ...
Another thing that happens often is that Cuban freelancers relocate in a country with no need for visa (with high speed internet connection) like Serbia or Ecuador and keep working with the client from there, with no need for higher salary.
... but overall, I think the opportunity to do some work you are proud is invaluable imo.
thanks for your time ... and sorry for my English
Now talking serious one friend of mine worked with Cuban Programmers. What he said:
"They have old and outdated machines so they are ninjas in optimizing the code to run faster"
It is not scientific but it was a first impression. Sometimes people from constrained countries HAVE to develop some skills that may be very useful for some companies.