I've not noticed this before but as it's specified in the pets resistance I would expect it to be an inherent -to hit effectively meaning a defense bonus.
That's how I read it anyway.
L50s: Tanks: Cryofission - Ice/EM - Dr Celsius - Fire/Ice - Saint George - SD/SS | Controllers: Psichosis - Ill/Kin - Major Chaos - Ill/Stm | Scrappers - Neutron Crusader - DM/SR
Currently Levelling: Angelic Blade - BS/WP Scrapper | Seeds of Destruction - Plant/Kin Controller
It's an Auto Power affecting the Caster (the pet) so yep, it's a tohit debuff. But given the very, very high survivability of Animated Stone and Singy it's a decent tradeoff.
Pets tend to hit fairly reliably anyway, especially with typical IO slotting.... allied +Tohit also isn't that hard to come by (Tactics, Fort, Forge etc.)
The earth primary does a ton of -def anyway so it's not been an issue for me.
Octavian Vanguard
@ohmsEU and @ohms 2
Badging character: Bimble on the Union server, Badgehunter.com and City Info Tracker.
Anyway it would be nice that in the pet power description you were told that the pet has lower than standard to-hit, so you can compensate with higher accuracy enhancement.
I also noticed recently that my warshade extracted essences seem to miss their targets more often, but I do not see lower to-hit in their descriptions.
In a related matter, it would be nice if, once and for all, the devs gave us a list of real auto-hit powers when it comes to pseudo-pets and pet-spawned pets. I am not really sure that powers like Trip Mines, burning Oil Patch, Burn (from Fire tankers), Acid Mortar and the like are REALLY auto-hit. 
None of those examples you've listed are autohit and never have been afaik. What made you think they were?
Octavian Vanguard
@ohmsEU and @ohms 2
Badging character: Bimble on the Union server, Badgehunter.com and City Info Tracker.
Some of those powers have the 100% accuracy that pets get so they seem more like autohit.
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Hey all!
I was doing a little reserch using City of Data for a controller build and found out that there are a couple controller pets (Animated Stone and Singularity) that list "To-Hit -25%"
among the effects of their Resistance power.
Does that mean that they have lower to-hit chance than other pets (and players): 50% instead of the usual 75%?
This To-Hit -25% does not appear in other pets like Fire Imps, and it's aldo not listed as "RES (To-Hit)" which would mean a resistance against To-Hit debuffs.