They probably say they don't support it because issues can happen between you and that seller. Or you could be given wrong info from that seller. It's not up to EA to smooth those issues that could happen out. It isn't up to them to advise you on if something goes wrong. If the key is wrong for instance you have no call to complain to them... you go back and deal with who you purchased it from.
Not supported doesn't mean it won't work it means they don't have to get involved if it doesn't for some reason. It means their support doesn't have intervene or "support" you with problems.
