Why are the Enterprise's away teams very lightly protected?

This bothers me sometimes. In this period of The Next Generation, offensive weapons (such as phasers) are very fatal. Humans and most other Starfleet personnel are very vulnerable to them. Yet when the Enterprise sends an away team, they are normally just sent on foot with minimal armor or protection, even when the planet may be hostile.

For example, in the episodes Descent (6x26 and 7x01), almost the entire crew were sent to the planet on foot, despite the obvious danger of being attacked by Borgs. Luckily of course, the Borgs only killed off minor characters :)

Is there any explanation of this policy? Even today, armies already make use of helicopters and armored vehicles when doing away missions (even peaceful ones like search and rescue, let alone dangerous ones).