Just to play devil's advocate, I think I know what part of the EULA that could be used to back his claim;
4. Acceptable Use Restrictions
4.1 You may not use the Game or any Online Features in any unlawful manner, for any unlawful purpose, or in any manner inconsistent with this EULA, or act fraudulently or maliciously, including but not limited to hacking into, inserting malicious code (including viruses or harmful data) into the Game, any Online Features or any operating system.
4.2 You may not infringe our intellectual property rights or those of any third party in relation to your use of the Game or any Online Features to the extent that such use is not licensed by this EULA.
4.3 You may not transmit any material that is unlawful, defamatory or offensive in relation to your use of the Game or the Online Features.
4.4 You may not use the Game or any Online Features in a manner that could damage, disable, impair, overburden or compromise our systems or security or interfere with the experience of other users of the Game or any Online Feature.
4.5 You may not collect or harvest any information or data from the Game, the Online Features or our systems, and may not attempt to decipher any transmissions to or from the servers running the Game or Online Features.
It could be argued that by blocking players while in Open Mode, you do "interfere with the experience of other users" - as other users would expect to see people who are in Open mode but not yet in a full "island" or "instance".
While, that may not be the intent of that part of the EULA, it is grey enough to be taken that way. Leaning to a moral question, should users be classed as cheaters if caught messing with external routers to control connections and use Open like it is Solo?
This thought did cross my mind with the race to Elite - what was stopping someone doing just that? And can FD prove it was done, to then refute a claim of Elite status gained in Open?
So, there we have it - not sure if that is what they were looking at - and I was bored, so thought I'd step to the dark side for 30 minutes.... they have cookies

lol
(I'm going for a shower now, I feel dirty after that

)