It tends to produce a treadmill, though--- since science funding is available, science profs are expected to get it. In some areas with less funding, it's perfectly normal for professors to have their students TA most of the time; but in most science departments, the prof's expected to pay a substantial proportion of their students from grant money, and it'll look bad if a prof is always "dumping students on the department" by funding them through TAships.
And if you're expected to pay your students, it takes a lot of money! One student, including tuition, stipend, and departmental overhead, costs around $50k, so if your lab is 5 students, you have to bring in $250k a year just to support your grad students. And if you start dipping below about $150k, so are supporting fewer than half your students, people will start grumbling, and it'll look bad for your tenure case. (You can't avoid it by having fewer students, either, because having only 2 students will also look bad for your tenure case.)