People will always exploit presentation, because presentation matters
I've gotten a number of plain-text emails lately that contained bold text (some of them were even non-spam emails). The innocent majority among you are wondering how on earth you get bold text into plain text email, while the unfortunate minority are nodding and sighing about Unicode bold characters. For those who haven't run into this before, Unicode defines a number of additional codepoints for mathematical symbols that happen to be bold versions of regular Latin letters. Since these are Unicode codepoints, you can put them into any Unicode or UTF-8 text, and then your email recipients are reading plain text email with Latin characters that their email client is probably going to render as bold.
(There are websites that will transform genuine plain Latin text into these Unicode codepoints for you, since it's a mechanical process.)
I'm not particularly surprised by the fact that people are doing this. Of course people are doing this, because people always do. If you give people a way of controlling the presentation of their work, even if it's a convoluted way, some number of them will take advantage of it. This is not because they're bad people; instead, it is because presentation matters to people. For text, a lot of the time that is specifically the visual appearance of the text in common environments. And so you have people using Unicode "Mathematical Bold" codepoints for Latin letters in email because it makes them look bold, and looking bold is important for the presentation that these people have in mind.
(The result is terrible for screen readers and other things that have to interpret the meaning of text, of course, but these are generally uncommon concerns in practice even if people should care about them in theory. If it becomes common enough, these things will bend to common usage, with mail client text search systems knowing that "Mathematical Bold" characters are the same as Latin letters and so on.)
You can expect any system or environment that provides a way to control presentation to have that way used sooner or later, because people always do it. No one should be surprised or horrified when it happens. My personal corollary is that if you want to steer people away from undesirable ways to control presentation, you should give them easy and good ways to do so.
(Presentation matters not just to the creator but also to the audience, because as people we really do judge things partly on their appearance, even if it's down in the depths of our brains. Creators know this, which is part of why they care about presentation.)