If you mean the error of “not found” kind, then that’s because those websites has been setup for years, and of course doesn’t have DNS problems. DNS problems always happens on new sites. This is unfortunately just how the Internet works. Just give it sometime and it will “act like a normal well behaved website”.
And if you don’t want others asking you why the site isn’t working, then maybe the next time you setup a new site, wait for several days before you rush to tell everyone about it?
If you mean the error of “deceptive site” kind, then that’s because those websites has been setup for years and is widely trusted. Websites using a free subdomain, on the other hand, tend to have the “noisy neighbour effect” where one bad site could affect the entire domain. This is how security system in browser works and unfortunately is not something we can control. You must bring up this to the corresponding company that is giving the error.