If they provide enough heat in the house/garage, why won't they provide enough heat in the greenhouse?
The heat loss from a glasshouse / polytunnel is very high ... a light in the corner of a garage won't be any better as the heat will escape into the rest of the garage itself, but if you build an insulated box in the garage then the heat will be retained (well ... very little top up heat will be needed, and the heat from the lamp may be enough). It is probably cheaper to light that box rather than heat the greenhouse and, in the early months, light in the greenhouse will be rubbish so heating it will encourage the plants into growth but they won't have the light to support that growth, lighting wins on that count too. I don't know if lighting is cheaper than heating on average ... clearly it would be in a very cold Spring.
A garage will not fall to as low a temperature, on a cold night, as a greenhouse as the heat loss through block walls is much slower than through glass. It won't heat up as much the following day either, but the extreme cold will be avoided. But it needs to be an insulated box if it is in a garage, in the house the minimum temperature will be whatever the central heating system maintains and how quickly the house loses temperature (overnight for example).
last year the ones I sowed in February in an unheated greenhouse without additional light seemed to do fine.
Sure, but last year (here at least) was incredibly mild. We had three frosts in the whole winter.
The likelihood is that Tomato seedlings subjected to cold will turn Purple, which is a sign of a defensive mechanism kicking in, the plant then fails to metabolise some key nutrients and gets stressed (or dies). It takes a considerable amount of time for the plant to get going again (presumably the defensive mechanism is evolved to assume that further cold nights may follow, until there is a prolonged period of warmth).
Yes indeed, you may well get away with it - maybe more often than not - but it is also likely that the stressed seedlings will be more susceptible to any diseases going around. You also have to look after them from February onwards, and experiments have shown that sowing that early only results in fruiting a few days earlier that plants from seeds sown in April - unless heat, and preferably light, is also provided.