My guess is that they see their device controlling a heating element of unknown (to them) provenance as a liability. My zen15 manual specifically has a warning against using it with heaters.

CAUTION This is an electrical device - please use caution when installing and operating the Power Switch. Remote control of appliances may result in unintentional or automated activation of power. Do NOT use this Z-Wave device to control electric heaters or other appliances which produce the risk of fire, burns, or electrical shock when unattended.

If they don't trust their specifications, do you? Pick up a Kill-a-Watt or some other way to measure them.

The only thing that comes to mind is that most resistive heaters eventually fail and might draw significant additional current when they do. But you should have plenty of extra margin with a 15A switch.

So I am definitely not an electrician by any means but I noticed on their site the ZEN05 says “Do not install the device in a place with direct sun exposure, high temperature, or humidity.” which seems pretty silly to me for something that two lines above says it is outdoor rated hah. I bet that is why they said not to use it though, heat+melting ice will generate humidity or something. I wouldn’t be placing the device next to the heating pad if it was me anyway so I would think it would work, your logic seems sound to me, I probably wouldn’t even have asked zooz now that I think about it.

That said, I’m going to need to hear more about these heating mats, I have some sidewalks that I don’t enjoy shoveling hah.

ETA: forgot to include, I use the GE version of that ZEN05 outside (since 2019) plugged into a GFCI outlet and it works fine, it definitely has very direct sun and gets all weather too.

Speaking from experience, Zooz products have been very poor quality for me and not worth the trouble. I put those switches all throughout my house and now I'm having to replace them, one by one, to due to random failures or unreliable performance. I used a Zooz switch for an outdoor outlet (switch was indoors) and the thing stopped working but support wouldn't do anything because it's not a light, it's going to an outlet. I supported them during their initial product launch, warrantied a lot of switches which helped their R&D, but customer support has 0 sympathy for me now that the warranty is over (they even disregarded the extended warranty they promised me). After all the work I've had to do tearing my outlets apart to replace their switches, it's just not worth it. I wish I would've gone with a more expensive brand the first time, I would have saved a lot. If it's not too late, I recommend looking beyond them.

I've had no issues with zooz products, if I had to guess I'd say they are trying to cover themselves from an automation starting a fire. Those two outdoor plugs are also not rated for below freezing, so the Enbrighten seems like a better fit.