Because we're living in an alternate universe where there's no such thing as VLANs?
[1] A handful because many of us are remote. Mine is a EeePC.
A VLAN would keep these computers on their own network, and firewalls could be set up on the network side to prevent this stuff from happening.
A MAC filter would do nothing in this situation, because you are using their computer. Even if you had a MAC filter, these computers would be white-listed anyway.