“Strange synthetics are usually harmless, emphasis on the “usually.” But organic elements can be really dangerous, where “really” means everyone dies horribly and nobody can ever go to the planet again.” (Network Effect Chapter 8)
Alien remnants are, as the name implies, archeological remains left by previous alien species. They can consist of inorganic compounds (“strange synthetics”) or organic remnants, which are considered much more likely to be hazardous.
The Strange Synthetics Accord ensures that special licenses are required to be allowed to work with any alien remnants, even within the Corporation Rim (where most restrictions are based solely on ability to pay) but because they are also extremely valuable, there is a high temptation to violate these regulatory agreements.