why SchemeBBS stops parsing at newlines
By design.
Users cannot be trusted for typography. They'll separate paragraphs with several lines (but SchemeBBS won't allow that) or they'll use typographical abominations such as text that's both bold and italic (super emphasis?)
Spoilers didn't even exist in the first version of the markup language (years ago, in the Erlang implementation). It's a concession made for the hypothetical poster from /prog/ who really can't live without them and a pain in the ass for those of us who don't use a mouse. You get to hide a word or two but you don't nest spoilers, you don't implement a Pong game in spoilers. No fun's allowed.
Nowadays you have only HTML generation, but that wasn't always the case. S-exp were translated to roff pages at one point. In a man page you have bold and underline but spoilers don't really exist (I know there's an ANSI escape code for invisible text). I digress. Try to do this in Markdown:
**
lol
**
What do you get? The rule of least surprise was followed, I guess.