Why can't I use my new pedal indoors? As seen below, I got a paper slip telling me not to use my new pedals on indoor equipment. I don't want to anyway, but I wonder — if threads would fit, is there any real reasons not to do it? Probably there is, because they wouldn't limit their own market without a cause, but what reason is it?

Silvery studs on pedals are fully removable.
As other have commented it appears to be based a legal opinion. While there is no physical reason they would not work the safety side says having sharp pins sticking out would not be a good idea. The potential for someone to strike a shin against the pedal warrants the warning.