'cos there's nowt wrong with them. While I'm aware tyres can degrade, mine haven't done and are still in "tip top" shape. It's probably safer to be on my old summer rubber in the summer than winter tyres.
I also eat eggs past their use by date as long as they don't float.
I drive year round on wintertires. This way I have fresh new winter tires every 3 years, instead of having two sets of 6 or more year old tires... I have done a trip to Venice with more than 30C weather, but never had any problems or noticed extra wear.
I've turned the air-bags off in my car as I've never used them so far and I haven't noticed a difference...
In extremis, the handling of your car will not be so good with hot winter tyres on a hot road. You probably won't realise until too late.
I wonder (and this isn't directed at you), how the Police/car Insurance would see it if you had an accident and they thought that having winter tyres fitted in Summer was a contributing factor?
You're most likely using quite a bit more fuel than you need to in summer, and tyre noise is louder and handling a little more sluggish than if you change to summer tyres in summer. Plus snow tyres absolutely do wear a lot faster than summer tyres, as they are made from a considerably softer compound. But if none of those consequences bother you, keep on using your snow tyres as all-season tyres. One advantage is improved handling and braking distances in wet weather, whatever the season... ETA, or not, if Tom's source (below) is correct...
Snow tyres are one of those things you need to consider mandatory in Switzerland even if officially they aren't, and don't even bother wondering why. Home insurance is another good example.
That article refers mainly to "winterreifen im sommer ausfahren" and "fast abgefahrenen Winterpneus".
IMHO: Using winter tyres with a good thread depth in summer is very different from driving almost used-up winter tyres in summer until there is no thread left.
Again IMHO: if you have relatively new winter tyres (>4mm thread left) and you drive few kms a year and well below the tyre's speed rating (e.g. max 140 vs. a 240kmh rating) and not in extremely hot temperatures (e.g. southern Italy) then your risk is not higher as compared to decent summer tyres.