Have you also noticed that many non-English speakers pronounce the adverb as just like ass? This embarrassing mistake is the result of not distinguishing between a voiced s (phonetic spelling [z]) and a voiceless s (phonetic spelling [s]). Even though as has two different pronunciations, referred to as weak and strong, the s is always a voiced s. The pronunciation of the strong form is [æz], that of the weak form is [əz]. You can listen to either of them here. For comparison, the proper pronunciation of ass (referring to either a body part or a furry animal) is [æs], listen here.
Interestingly, the different pronunciation of as and ass is routinely missed by German speakers, even though the German language also uses both the voiced and the voiceless s. Austrians like myself have it a bit harder because [s] and [z] are often interchangeable in Austrian German.