Was just going to say thats racist either way...well..not so much as racist, but xenophobicPartially, but, er.
Japanese people to put it bluntly don't like outsiders and the people who learn Japanese put a lot of effort in to assimilate which also tends to unfortunately involve proving that you're "one of the good ones".
We're talking about a country that is still debating whether or not it's ethical to force women (and only women) to wear high heels, get contacts or eye surgery because glasses are not part of one's uniform as well as one that forces children to dye their hair if they cannot get a doctor's note that states they are not genetically Japanese. These are not exaggerations and I have proven the women's worker rights multiple times on these forums.
Another example is that only recently did several schools announce that they won't force children to dye their hair or to wear specific colours of undergarments... but these are just a few, specific schools. The vast majority still do or at least have the power to continue to do so.
Even when one goes through the process to become a Japanese citizen they don't take your race or previous country into account during their census. They just mark you down as being Japanese as even the system is designed around everyone fitting a particular mold.
Breaking the mold is frowned upon. You, as a foreigner, break the mold by default. So they just get ornery and annoyed. I assure you it's less cultural differences and just ol' fashioned racism, as much as the weebs would scream otherwise.
As long as you try your best to be respectful and to write in the best Japanese you can, you're welcome to post there; there's no rule stating otherwise. People who've learned Japanese just try really hard to prove they're "not like the other foreigners" which to me is a waste of time and energy since anyone who is racist is just gonna lump you in with them either way.