Why is there a distinction? The 3G wireless network as you note in your question is much slower. It's also much more crowded than the new 4G network. Since Verizon services more customers on a network that doesn't have as much capacity, there is more opportunity for congestion.
Depending on how congested the network is your service could be slowed for a few minutes or for several hours.
Keep in mind this policy only applies to customers with unlimited data plans on the 3G network. Verizon doesn't slow down data for 4G customers.