Why can't I use my new pedal indoors?

My indoor exercise cycle specifically cautions against using pedals which are not designated as being strong enough for indoor cycles. Supposedly, the stresses put on pedals on exercise bikes are greater than those on real bikes.

I believe this has nothing to do with the pins, or with any special mechanical stress that indoor cycling involves.

In my opinion the issue is the possibility of corrosion due to sweat. Indoor bikes get a lot more sweat on them because they don't move. It's a huge problem that can quickly ruin components and even create the possibility of a sudden and dangerous failure.

https://cyclingmagazine.ca/sections/training-guide/training/save-you-bike-from-sweat/

I bet the pedal manufacturer's legal department is worried that they never specifically tested for breakage due to corrosion from all that added salt. It's probably fine, but without specific testing they are wise to avoid any guarantees.

Probably due to the fact that many indoor bikes, particularly spin bikes, are fixed wheel inc. a heavy flywheel which is why they always have toe-clips and/or clip in pedals - to make sure foot says on the pedal.

If your foot slipped off with pinned flats, the pedals will still be whizzing round and then potentially cause an injury.

Here's an extract from a warning sticker from a spin bike:

Flywheel momentum will keep pedals turning even after you stop pedaling. Do not attempt to dismount bike or remove feet from pedals until pedals have completely stopped.