The rules for FTP are somewhat specific, as it is necessary for them to be. FTP links in HTML documents are often included for purposes of reference, and it is often inconvenient to download them by default.
To have FTP links followed from HTML documents, you need to specify the ‘--follow-ftp’ option. Having done that, FTP links will span hosts regardless of ‘-H’ setting. This is logical, as FTP links rarely point to the same host where the HTTP server resides. For similar reasons, the ‘-L’ options has no effect on such downloads. On the other hand, domain acceptance (‘-D’) and suffix rules (‘-A’ and ‘-R’) apply normally.
Also note that followed links to FTP directories will not be retrieved recursively further.