Homebrewer You are correct about plugin gridtie. If someone told you that the device has a UL cert and is a US market plugin gridtie inverter, you should go here:
http://ul.com/offerings/manufacturers/global-security-brand-protection/contact-ul-gsbp/ and report it.
UL would not certify anything that is forbidden under the NEC. The reason for forbidding a plugin gridtie are:
1. The meter would have to be changed for your backfeed to register properly. Modern meters cannot run backwards to prevent someone from flipping the meter over to get free electricity as was possible in the past. You either get a one-way meter (asymetric) or you have a proper grid-tie meter installed after the installation has been inspected. If you force an asymetric meter to run backwards you will be charged for the electricity going out to the grid and the meter will set a flag that indicates you tampered with it.
2. The power company wants to know where a potential backfeed to the grid is, so if the need to work on the lines or transformers, they can lock out and isolate the gridtie to protect their lineman. Gridtie inverters are not supposed to backfeed if the grid is down, but smart people don't take chances.
3. A plugin inverter is a safety hazard in more than one way.
A. Consider the cord used. Most of the illegal gridtie inverters I have seen use a common computer/appliance cord. Those cords come in different lengths and wire gauge, but they are all physically interchangeable. How would you insure that the cord you choose can handle the load?
B. Where are you plugging the portable inverter in? An outlet somewhere. Most likely an outlet on a branch circuit. You are not backfeeding the grid as much as you are backfeeding a branch somewhere in your home's electrical system.
C. Most new home come with ground fault circuit interrupters. GFCI's have a "line" and "load" side. If you have one at the service panel and try to backfeed a branch out you will smoke the panel GFCI. Big bucks to replace. If you try to backfeed an individual GFCI it will fail. If you remove an individual GFCI to backfeed that branch you will have no line fault protection on that branch.
D.Proper gridtie inverters backfeed symetrically. In the US we have split phase 240 for residential service. The transformer on the pole is center tapped to ground and the center tap is also the neutral. Each half of the transformer feeds 120 to one side or the other of your service panel. Use one side hot and you have 120v, use both hot wires and NO neutral and you have 240v. If you backfeed with a plugin 120v inverter you can only feed one half of the transformer. Also the only place the ground and neutral are bonded is at the pole and at the panel. Your 120v inverter won't work properly plugged in a branch because there are stray currents and voltage present on branch circuits from less than perfect power factors of appliances. A gridtie inverter installed at the panel will see a clean ground and neutral because they are bonded there and your panel is earthed there.