I can't believe the reaction which the origional post has recieved. Its just a peice of (good) advice. I live just a few miles from the section of A30 in question (near Truro), and having done numerous cycle tours across the UK and Europe I would advise NOT to use this section of road. There are plenty of alternative roads in the area - they just require a little planning and possibly an extra day on your tour.
Yes, you SHOULD be able to ride on any section of road you like, in complete safety, but the situation is what it is - the A30 is very busy, often narrow and has people driving at motorway speeds: Not suitable surroundings for most cyclists. Thats simply the advice.
As someone else said, the stakes are much higher. You get hit on a lane at 30mph, you may be OK. you get hit on the A30 by something moving at 70mph, you've had it.