It's probably simply a matter of economics. S4C does provide English subtitles for the majority of its programmes:
http://www.s4c.cymru/en/access-services/page/5854/english-subtitles/
However to provide subtitles for a rugby match they'd need to have two 'revoicers' to provide simultaneous translation between Welsh and English. People capable of providing simultaneous translations (i.e. who're listening to 'Sentence 2', and translating it in their heads, as they're speaking the translated version of 'Sentence 1') tend to charge very high fees, with £200 per hour being a typical starting rate. However 'revoicers' also need to have additional skills for subtitling, perhaps pushing their rates up to £300 per hour. With two of them working '15 minutes on and 15 minutes off', that would put S4C's costs up to £600 per hour. The audiences for many Clwb Rygbi programmes will be very small and those for Rygbi Pawb even smaller, meaning that the additional advertising revenue they might get from attracting a few non-Welsh speakers to those audiences wouldn't justify the cost of providing translation/subtitling services.
I'm not really sure why your post refers to 'BBC subtitling' and paying your licence fee, as S4C isn't part of the BBC, as such. (The BBC provides some programming content, such as new bulletins, free of charge to S4C to enable it to meet its charter obligation to provide services to Welsh-speakers. It also provides much of S4C's funding but only on an 'arm's length' basis. S4C is still a separate broadcaster).
For more information about the way that subtitling is produced, please refer to my post here:
https://www.theanswerbank.co.uk/Media-and-TV/Question1677564.html