Well....basically you purchased a phone designed to work on Verizon's network as it is a Verizon CDMA phone. But it can also run on ATT or Tmobile networks because it has the radios inside to do so.
Your only other choice would be to get a non-Verizon branded phone to use on Tmobile if you do not want to look at that splash screen or the logo under the glass and on the back.
Its kind of like you brought a subway sandwich into a mcdonalds and put a few quarter pounder paddies inside that subway sandwich....but its still a subway sandwich....haaa
MVNO's are prepay providers that buy blocks of use with Verizon, ATT, Tmobile, Sprint. Like Boost, Cricket, Straight Talk, Page Plus, Total Wireless, Selectel, etc. I use PagePlus on my Icon with no problems using the same voice network but with a speed caped data ability. Wich is not a problem as I do not use data for streaming or have needs for large fast amounts of mobile data on my phone. I pay less than $30 per month for `1200 mins, 3000 texts, and 500mb of date use with no contracts and can bail at any time. This is why people use MVNO's......cheaper and easy to try different ones when you want. Also, it all depends on your coverage area with each choice of the main providers. I could not make it with Tmobile as they do not have good coverage in my area. So I use a Verizon mvno service.