be against当中的against是介词，为什么前面还是be动词？

因为 against 后还应该带有宾语构成介词短语。介词短语可以充当状语、后置定语和表语。be against ... 就是用介词短语。而如果要用介词短语的话，前面必须加上be动词。
例如：
It is against our policy.
这和我们的方针相抵触。

This is against the law.
这是违法的。

They are disinclined to use violence because it is against their Buddhist faith.
他们不愿意使用暴力，因为这么做违背他们的佛教信仰。

You can't do anything of the sort. It is against orders.
你不能干这事，这是违反命令的。

Judging from his tone, there's no doubt he is against it.
听他的话音，便知他是反对的。

Concealment of evidence is against the law.
隐藏证据是违法的。

against 英[əˈgenst] 美[əˈɡenst]
prep. 反对; 对…不利; 紧靠; 以防;
[例句]She leaned against him.
她紧靠着他。

against 后还应该带有宾语构成介词短语，介词短语可以充当状语、后置定语和表语，be against ... 就是用介词短语 “against ...” 作表语。

因为介词不能单独作谓语，故与be一起构成系表结构！

因为against不能作谓语，所以这里用be动词作谓语