In this syntactic context "for" is considered usually to be a conjunction of subordination (meaning "because", not used much except in literary text), although some will call it rather a conjunction of coordination.
There is a serious argument for classifying "for" as a subordinating conjunction (CoGEL § 13.18).
13.18 Coordination-subordination gradient
Table 13.18 displays the gradient from the central coordinators and and or to subordinators like if and because, with but, for, and so that on the gradient. The conjuncts yet, so, and nor are added to the Table, because, as we have seen, they in some respects resemble coordinators. The six features of and and or noted in 13.7-17 have provided six criteria used in constructing the matrix. If an item satisfies a criterion, this is indicated by a '+' in the relevant cell. If it fails to satisfy the criterion, '-' is entered. The combination '±' takes care of cases, explained in the previous discussion, where the item satisfies the criterion only under certain conditions. The six criteria to be applied to each item are :
(a) It is immobile in front of its clause.
(b) A clause beginning with it is sequentially fixed in relation to the previous clause, and hence cannot be moved to a position in front of that clause.
(c) It does not allow a conjunction to precede it.
(d) It links not only clauses, but predicates and other clause constituents.
(e) It can link subordinate clauses.
(f) It can link more than two clauses, and when it does so all but the final
instance of the linking item can be omitted.
Table 13.18 Coordination-conjunct-subordination gradients
(a) |
(b) |
(c) |
(d) |
(e) |
(f) |
|
|
+ |
+ |
+ |
+ |
+ |
+ |
coordinators |
and, or |
+ |
+ |
+ |
+ |
± |
- |
|
but |
+ |
+ |
x |
+ |
- |
- |
conjuncts |
yet, so, nor |
- |
+ |
- |
- |
- |
- |
|
however, therefore |
+ |
+ |
+ |
- |
- |
- |
subordinators |
for, so that |
+ |
± |
- |
- |
- |
- |
|
if, because |
It can be seen that the criteria that make "and" and "or" true coordinators fail in the case of "for". "For" is therefore more akin to because", which is definitely at the subordinator end of the gradient.
(CoGEL§2.60)
[...] there is a scale relating coordinating and subordinating conjunctions, such that and and if represent clear cases of each category, whereas for is in an intermediate position:
COORDINATING SUBORDINATING
and<--------------for------------>if
Nevertheless, for is closer to if in its syntactic behaviour than to "and" , and can reasonably be classed as a peripheral subordinator.
Note "For" is not a preposition because the prepositional complement is "characteristically a noun phrase, a nominal wh-clause or a nomial -ing clause" (CoGEL § 9.1). There is not a complement in what follows "for"; instead there is a finite verb form ("is"), which is the mark of a finite clause; this syntax can only justify "for" as a subordinator (or a coordinator, according to those who ould rather treat it as such).